Xuldarinar
2014-10-08, 11:23 PM
BECOMING A DEMON: THE GUIDE (?)
Do you want to become a demon? Who doesn't? Lets get started.
Now, take this guide with a grain of salt. I don't claim to know everything and I do make mistakes periodically. If you see any mistakes made, any information omitted, then by all means please correct me. I'll go as deep as I reasonably can, and almost certainly will expand upon things periodically.
The Ritual
Knowledge of the ritual is obscure at best, hidden away in rare tombs and secrets kept by a wide range of demons. To begin on your journey, you must know about the journey itself and what it takes. There are three ways individuals generally learn about the ritual and how to preform it.
You have found a rare tome, such as the Book of the Damned, that possesses documentation regarding the ritual. Whether you are fortunate to stumble upon it, or you actively seek it out, that is left to chance. This can be done at any level with any. You might have even stumbled upon such a tome before 1st level.
You have a demon placed in your service who happens to know the ritual. All left to chance for antipaladins who choose the service of a fiend for their boon, those that may obtain familiars who take on a quasit for their familiar, or even others still. In any case, this demon might know about the ritual and just might be more than happy to teach you. For familiar classes, this can be done at the soonest at 7th level. For antipaladins, this opportunity opens up at 5th level and a new chance every time they select a different demon for their boon. Other classes, and archetypes of classes, may have similar opportunities as well.
You seek out the information yourself, not in tomes directly but for demons who know the ritual. Whether by chance in casting the right spells, or specifically summoning the right one, you may bargain with them for them to teach you the way. Must be able to cast Planar Ally (Cleric 11, Oracle 12, Witch [Spirits Patron] 16, Warpriest 16) or Planar Binding (Wizard 11, Sorcerer 12, Summoner 13, Witch [Dimensions Patron] 12), and bind a demon that knows of the transformation ritual. Keep in mind, you may be able to do this earlier by getting another spellcaster who is capable of using the proper spell, but it is certainly better if you can cast the spells yourself.
The First Ritual
First Ritual: The first ritual is the easiest, and requires nothing more than for the user’s alignment to become chaotic evil. By undertaking acts of a vile and destructive nature, the mortal consigns her soul to the Abyss even if she never finishes the second and third rituals. The mortal must select a demon lord (or a nascent demon lord or Balor Lord) at this point to serve as a demonic patron. The acts the user must undertake can vary, but should be of a nature that represents and “honors” the areas of interest of her chosen demonic patron—these acts are intended to attract the patron’s attention, after all. Even mortals who begin the transformation ritual as already chaotic evil must undergo this stage of the ritual, if only to select a demonic patron. In total, the first ritual must progress in this manner for a minimum of one year—at the end of the ritual, the mortal’s alignment changes to chaotic evil.
Yep. Once you know of the ritual, you need to be chaotic evil and pick a demon lord, a nascent demon lord, or a balor lord. This will be relevant later on, effecting your conduct on your path to demonhood, as well as having some impact on what type of demon you can become. Worship an incredibly powerful demon for a year, and even if you weren't chaotic evil before, by the end of the year you are chaotic evil. If you need help deciding who to serve, or want even just a list of options, have a look at this (http://www.archivesofnethys.com/DeitiesByGroup.aspx), and scroll down to the demon lords.
The Second Ritual
Second Ritual: The second ritual requires many more months of debased acts and vile plots, but at some point during this second year, the mortal must contact her chosen demonic patron, via either commune or contact other plane. A trusted minion or ally can cast this spell on the mortal’s behalf—it’s common for a spellcaster to gain a quasit familiar so that the quasit can use its commune spell-like ability for this purpose. Once contact is made, the demonic patron must be told of the mortal’s desire to become a demon—the commune or contact other plane spell cannot be used to ask any other questions. At some point thereafter, but before a year has passed, the mortal must offer a significant sacrifice to her demonic patron as a burnt offering in a temple consecrated to the demon lord. This offering can be either living or dead—in either case, the burnt offerings must be of someone that is related to the mortal by blood or family—an adopted parent or child is sufficient if, at some point the mortal and the offering once loved each other. The mortal must make a DC 20 Knowledge (religion) check to properly perform the burnt offering—if the sacrifice is or was once a worshiper of a lawful or good deity, the check drops to DC 10 (and thus becomes something that even one not trained in the Knowledge skill can attempt). If this check is successful, the second ritual ends and the mortal gains a demonic implant (typically demon senses or a demon talon) as proof of her demon’s favor.
Now some spells and knowledge will be put to the test, and you get to cook up a member of your family. If you have never loved anyone and don't know who your family is, then you are out of luck. If your family is dead, get a shovel. In any case, you have a time limit from the time you have the spell cast, but a year shouldn't be a problem to get this done within. As a reward, you get one of these (http://www.archivesofnethys.com/MagicDemonicImplants.aspx), and can move onto the third step.
Classes that have knowledge (religion) as a class skill and/or tend to have high intelligence scores will naturally have an easier time making the check from this. Skill Focus (Knowledge [Religion]) will help out, now and for later stages, a great deal. They aren't required, but it will save time and hassle. If the family member you use as an offering was once a worshipper of a lawful or good deity, you may never have to put any skill points into knowledge (religion) to complete your journey.
The Third Ritual
Third Ritual: At some point after the mortal contacts her demon lord (typically after the second ritual succeeds, but sometimes before), the demon sends the mortal a vision of a task that must be completed (such as freeing a bound evil outsider from a Material Plane prison or assassinating a powerful cleric of a good religion). This task is typically one of significance to the demon lord, and in many cases one that the mortal has no hope of completing until she grows more powerful. There is no time limit for how long the mortal has to complete this task, but she must maintain her weekly devotions to her demon in the time that passes in the form of regular worship and continued atrocities in the demon’s name. Additional burnt offerings to the demon, betrayals of allies, and assaults on innocents are popular choices. Once the assigned task is completed, the mortal must perform another ritual in honor of her demonic lord—a sacrifice consisting of no less than a dozen nonevil intelligent creatures offered in the course of no more than a single week. The mortal must make a DC 30 Knowledge (religion) check at the end of the sacrifice, but gains a +1 bonus on the check for every 5 additional sacrifices offered beyond the initial dozen. If the check succeeds, the third ritual ends and the mortal permanently gains the half-fiend template.
Now we get serious. You have a quest to complete, so go do it. You don't have a hard time limit, but as soon as it is in your capability to complete it should be dealt with. Then, you have a another sacrifice to make and with it a knowledge check. If you first don't succeed, try try again. Even if you have no ranks in knowledge (religion), if not before then certainly by the time you've sacrificed upwards of 300 people over the course of this stage you'll get it. While I do recommend you do what you can to increase your skill checks in regards to knowledge (religion), there are advantages to completing the last stage of the ritual later rather than sooner. I advise that you put one of your +4 bonuses from becoming a half-fiend (http://www.d20pfsrd.com/bestiary/monster-listings/templates/half-fiend) into Int. It won't matter after you become a demon, if you decide to carry this out to the bitter end, but it will help you get there.
The Fourth Ritual
Fourth Ritual: For many, gaining the half-fiend template is enough, but for those who wish to go even further, a fourth ritual exists. The mortal’s demon lord grants no vision or advice to begin this ritual—the mortal must take it upon herself to honor her demon lord in a manner appropriate to that demon lord’s interests and areas of concern. Once every year, on the anniversary of the day the mortal completed the third ritual, she must perform a special ceremony that recounts her accomplishments over the past year and culminates in a significant sacrifice (usually consisting of the sacrifice of an important member of an enemy faith, or of a lawful or good outsider; in either case, the sacrifice must be at least CR 9). The mortal then makes a DC 40 Knowledge (religion) check, with a cumulative +1 bonus for each previous such ritual she’s performed over the past several years. Success results in a final transformation into a full-fledged demon—the mortal loses all benefits of her previous race or the half-fiend template, but can immediately apply all of her class levels to her new demonic race (for example, a human fighter 10 could become a vrock fighter 10). The type of demon that the mortal transforms into depends on both the nature of her chosen demon lord and the GM’s discretion, but should generally not result in a total number of Hit Dice that more than doubles the mortal’s original Hit Dice.
Congratulations for making it this far. To complete your transformation, you gotta wait a year and then make an appropriate sacrifice. If you preform it well enough, you can leave mortality behind. If not, there is always next year.
Now, exactly how this functions is based on your reading of it, post ritual of course. My reading goes as follows.
Drop your racial traits and the benefits of the half-fiend template and then add pretty much everything the demon has short of equipment to you. However, you can interpret it such that you drop one and keep the other. If you follow this path, keep in mind if you maintain your racial traits, you should treat any discrepancies (such as type) to take after the Demon you become. For example; An elven balor would most likely be Large Outsider (Chaotic, Demon, Elf, Evil, Extraplanar). Your ability scores will receive bonuses based on the demon type (-10 to all even ability scores, -11 to all odd ability scores. Add these as bonuses to what your ability scores are after you take off your former race's traits and the template.)
The Demons
Now, the important question is, after all that work what demon do you become? Well, it does hinge on a few things. Your HD upon the completion of the fourth ritual, the demon you serve, and potentially the things you have done along the way. To make things easier, here is a table containing all the demons I could find, organized by HD, along with the modifiers they would offer. Remember, you cannot become a type of demon that would more than double your HD by becoming one. In essence, you generally cannot become a demon that has more HD than you do at the completion of the fourth ritual.
Do take this list with a grain of salt, as some entries may be absent (especially with the addition of new demons and existence of 3rd party content.)
HD Type of Demon Ability Score Modifiers
2 Dretch (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/dretch)+2 Str, +2 Con, -6 Int
3 Quasit (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/quasit)-2 Str, +4 Dex, +2 Wis
4 Vermlek (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vermlek)+4 Str, -2 Dex, +4 Con, +2 Int, +2 Wis, +2 Cha
5 Abrikandilu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-abrikandilu)+4 Str, +2 Con, -4 Int, +2 Cha
5 Hala (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-hala)+6 Str, +8 Dex, +4 Con, +2 Int, +4 Cha
5 Schir (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-schir)+6 Str, +4 Dex, +4 Con, -2 Int, -6 Wis, -4 Cha
5 Swaithe (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-swaithe)+6 Dex, +4 Con, +2 Int, +2 Cha
6 Brimorak (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-brimorak)+6 Str, +6 Dex, +8 Con, +2 Int, +2 Wis, +4 Cha
7 Babau (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/babau)+10 Str, +2 Dex, +10 Con, +4 Int, +2 Wis, +6 Cha
7 Shadow (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-shadow)Str -, +8 Dex, +6 Con, +4 Int, +4 Wis, +8 Cha
8 Incubus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-incubus)+10 Str, +4 Dex, +8 Con, +6 Int, +4 Wis, +10 Cha
8 Succubus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/succubus)+2 Str, +6 Dex, +10 Con, +8 Int, +4 Wis, +16 Cha
9 Nabasu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/nabasu)+12 Str, +6 Dex, +12 Con, +4 Int, +6 Wis, +8 Cha
9 Vrock (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/vrock)+10 Str, +4 Dex, +14 Con, +4 Int, +6 Wis, +6 Cha
10 Hezrou (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/hezrou)+16 Str, +18 Con, +4 Int, +4 Wis, +8 Cha
10 Kalavakus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-kalavakus)+12 Str, +2 Dex, +14 Con, +4 Int, +6 Wis, +6 Cha
11 Derakni (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-derakni)+16 Str, +12 Dex, +12 Con, -2 Int, +6 Wis, +6 Cha
11 Gibrileth (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-gibrileth)+14 Str, +8 Dex, +14 Con, +2 Int, +6 Wis, +6 Cha
11 Kithangian (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-kithangian)+12 Str, +10 Con, -4 Int, +8 Wis, +6 Cha
11 Shachath (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-shachath)+14 Str, +14 Dex, +16 Con, +8 Int, +10 Wis, +10 Cha
11 Xenarth (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-xenarth-ichor-shark)+18 Str, +4 Dex, +16 Con, -4 Int, -4 Cha
12 Glabrezu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/glabrezu)+20 Str, +20 Con, +6 Int, +6 Wis, +10 Cha
12 Omox (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-omox)+16 Str, +14 Dex, +14 Con, +4 Int, +8 Wis, +8 Cha
14 Nalfeshnee (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/nalfeshnee)+22 Str, +2 Dex, +18 Con, +12 Int, +12 Wis, +10 Cha
14 Oolioddroo (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-oolioddroo)+4 Str, +14 Dex, +14 Con, +10 Int, +14 Wis, +12 Cha
14 Painajai (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-painajai)+16 Str, +6 Dex, +18 Con, +8 Int, +8 Wis, +8 Cha
15 Seraptis (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-seraptis)+12 Str, +12 Dex, +18 Con, +6 Int, +8 Wis, +10 Cha
16 Coloxus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-coloxus)+8 Str, +18 Dex, +10 Con, +6 Int, +8 Wis, +12 Cha
16 Marilith (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/marilith)+14 Str, +8 Dex, +22 Con, +8 Int, +8 Wis, +14 Cha
17 Lilitu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-lilitu)+16 Str, +10 Dex, +20 Con, +10 Int, +12 Wis, +16 Cha
17 Shemhazian (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-shemhazian)+24 Str, +8 Dex, +18 Con, +16 Wis, +6 Cha
18 Vavakia (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vavakia)+22 Str, +4 Dex, +22 Con, +8 Int, +10 Wis, +12 Cha
18 Vilsteth (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vilsteth)+12 Str, +8 Dex, +14 Con, +12 Int, +6 Wis, +12 Cha
19 Gallu (Warmonger) (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-gallu)+22 Str, +8 Dex, +24 Con, +8 Int, +12 Wis, +14 Cha
19 Vrolikai (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vrolikai)+20 Str, +12 Dex, +24 Con, +12 Int, +12 Wis, +16 Cha
20 Balor (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/balor)+24 Str, +14 Dex, +26 Con, +14 Int, +14 Wis, +16 Cha
The Price of Power
The higher level you are, the greater your options are when it comes to demons. One could argue you should go for the most powerful demon available to you. After all, you have the most to benefit in the moment, but this comes with its own drawbacks. For one, you are effectively spontaneously gaining anywhere from 2 to 20 levels, and although in each set the benefits are fixed, still the more levels you gain the more powerful than the rest of the group you may become. Furthermore your advancement will slowed down. For classes and characters that don't depend upon the advancement of particular abilities have the most to gain. They won't fall behind in that area as easily, and their might is easily added to your own. For spellcasters, those that have the easiest time becoming demons, they will find that their slowed advancement for being several levels ahead may cripple them in the long run, pending upon how much longer the campaign continues. There is nothing wrong with seeking to become a balor or the 3rd party choronzon, but don't forget there are benefits even for becoming the lowly quasit.
But don't become a dretch. Nobody likes dretches.
Classes that Shouldn't
Unfortunately, becoming a demon is not for everyone. There are some classes that should be addressed when considering undertaking the process.
Druid: While against the natural order of things, druids are among the more common groups that have contention with demons to worship demons or even consider becoming them. Regretfully, given the nature of the process you will be an ex-druid for some time if you go down this path. If after you manage to balance yourself between law and chaos, or good and evil, you may then return to being a druid.
Monk: Most monks require discipline to continue advancing through their training. So unless you are simply a martial artist, keep in mind the fact that once you start down the road to demon-hood, your training will have to be put on hold indefinitely. Fortunately, you retain your abilities.
Paladin: Paladins are directly opposed to demon-kind. If you want to become one, you must stop being a paladin. I advise considering becoming an anti-paladin, so at least your training doesn't go to waste. If you intend to return to being a paladin afterwards, or wish you could retain your paladin abilities while you follow this path, this is not the guide for you. The road to damnation is swift, and the road to redemption once there can be impossibly steep.
Do you want to become a demon? Who doesn't? Lets get started.
Now, take this guide with a grain of salt. I don't claim to know everything and I do make mistakes periodically. If you see any mistakes made, any information omitted, then by all means please correct me. I'll go as deep as I reasonably can, and almost certainly will expand upon things periodically.
The Ritual
Knowledge of the ritual is obscure at best, hidden away in rare tombs and secrets kept by a wide range of demons. To begin on your journey, you must know about the journey itself and what it takes. There are three ways individuals generally learn about the ritual and how to preform it.
You have found a rare tome, such as the Book of the Damned, that possesses documentation regarding the ritual. Whether you are fortunate to stumble upon it, or you actively seek it out, that is left to chance. This can be done at any level with any. You might have even stumbled upon such a tome before 1st level.
You have a demon placed in your service who happens to know the ritual. All left to chance for antipaladins who choose the service of a fiend for their boon, those that may obtain familiars who take on a quasit for their familiar, or even others still. In any case, this demon might know about the ritual and just might be more than happy to teach you. For familiar classes, this can be done at the soonest at 7th level. For antipaladins, this opportunity opens up at 5th level and a new chance every time they select a different demon for their boon. Other classes, and archetypes of classes, may have similar opportunities as well.
You seek out the information yourself, not in tomes directly but for demons who know the ritual. Whether by chance in casting the right spells, or specifically summoning the right one, you may bargain with them for them to teach you the way. Must be able to cast Planar Ally (Cleric 11, Oracle 12, Witch [Spirits Patron] 16, Warpriest 16) or Planar Binding (Wizard 11, Sorcerer 12, Summoner 13, Witch [Dimensions Patron] 12), and bind a demon that knows of the transformation ritual. Keep in mind, you may be able to do this earlier by getting another spellcaster who is capable of using the proper spell, but it is certainly better if you can cast the spells yourself.
The First Ritual
First Ritual: The first ritual is the easiest, and requires nothing more than for the user’s alignment to become chaotic evil. By undertaking acts of a vile and destructive nature, the mortal consigns her soul to the Abyss even if she never finishes the second and third rituals. The mortal must select a demon lord (or a nascent demon lord or Balor Lord) at this point to serve as a demonic patron. The acts the user must undertake can vary, but should be of a nature that represents and “honors” the areas of interest of her chosen demonic patron—these acts are intended to attract the patron’s attention, after all. Even mortals who begin the transformation ritual as already chaotic evil must undergo this stage of the ritual, if only to select a demonic patron. In total, the first ritual must progress in this manner for a minimum of one year—at the end of the ritual, the mortal’s alignment changes to chaotic evil.
Yep. Once you know of the ritual, you need to be chaotic evil and pick a demon lord, a nascent demon lord, or a balor lord. This will be relevant later on, effecting your conduct on your path to demonhood, as well as having some impact on what type of demon you can become. Worship an incredibly powerful demon for a year, and even if you weren't chaotic evil before, by the end of the year you are chaotic evil. If you need help deciding who to serve, or want even just a list of options, have a look at this (http://www.archivesofnethys.com/DeitiesByGroup.aspx), and scroll down to the demon lords.
The Second Ritual
Second Ritual: The second ritual requires many more months of debased acts and vile plots, but at some point during this second year, the mortal must contact her chosen demonic patron, via either commune or contact other plane. A trusted minion or ally can cast this spell on the mortal’s behalf—it’s common for a spellcaster to gain a quasit familiar so that the quasit can use its commune spell-like ability for this purpose. Once contact is made, the demonic patron must be told of the mortal’s desire to become a demon—the commune or contact other plane spell cannot be used to ask any other questions. At some point thereafter, but before a year has passed, the mortal must offer a significant sacrifice to her demonic patron as a burnt offering in a temple consecrated to the demon lord. This offering can be either living or dead—in either case, the burnt offerings must be of someone that is related to the mortal by blood or family—an adopted parent or child is sufficient if, at some point the mortal and the offering once loved each other. The mortal must make a DC 20 Knowledge (religion) check to properly perform the burnt offering—if the sacrifice is or was once a worshiper of a lawful or good deity, the check drops to DC 10 (and thus becomes something that even one not trained in the Knowledge skill can attempt). If this check is successful, the second ritual ends and the mortal gains a demonic implant (typically demon senses or a demon talon) as proof of her demon’s favor.
Now some spells and knowledge will be put to the test, and you get to cook up a member of your family. If you have never loved anyone and don't know who your family is, then you are out of luck. If your family is dead, get a shovel. In any case, you have a time limit from the time you have the spell cast, but a year shouldn't be a problem to get this done within. As a reward, you get one of these (http://www.archivesofnethys.com/MagicDemonicImplants.aspx), and can move onto the third step.
Classes that have knowledge (religion) as a class skill and/or tend to have high intelligence scores will naturally have an easier time making the check from this. Skill Focus (Knowledge [Religion]) will help out, now and for later stages, a great deal. They aren't required, but it will save time and hassle. If the family member you use as an offering was once a worshipper of a lawful or good deity, you may never have to put any skill points into knowledge (religion) to complete your journey.
The Third Ritual
Third Ritual: At some point after the mortal contacts her demon lord (typically after the second ritual succeeds, but sometimes before), the demon sends the mortal a vision of a task that must be completed (such as freeing a bound evil outsider from a Material Plane prison or assassinating a powerful cleric of a good religion). This task is typically one of significance to the demon lord, and in many cases one that the mortal has no hope of completing until she grows more powerful. There is no time limit for how long the mortal has to complete this task, but she must maintain her weekly devotions to her demon in the time that passes in the form of regular worship and continued atrocities in the demon’s name. Additional burnt offerings to the demon, betrayals of allies, and assaults on innocents are popular choices. Once the assigned task is completed, the mortal must perform another ritual in honor of her demonic lord—a sacrifice consisting of no less than a dozen nonevil intelligent creatures offered in the course of no more than a single week. The mortal must make a DC 30 Knowledge (religion) check at the end of the sacrifice, but gains a +1 bonus on the check for every 5 additional sacrifices offered beyond the initial dozen. If the check succeeds, the third ritual ends and the mortal permanently gains the half-fiend template.
Now we get serious. You have a quest to complete, so go do it. You don't have a hard time limit, but as soon as it is in your capability to complete it should be dealt with. Then, you have a another sacrifice to make and with it a knowledge check. If you first don't succeed, try try again. Even if you have no ranks in knowledge (religion), if not before then certainly by the time you've sacrificed upwards of 300 people over the course of this stage you'll get it. While I do recommend you do what you can to increase your skill checks in regards to knowledge (religion), there are advantages to completing the last stage of the ritual later rather than sooner. I advise that you put one of your +4 bonuses from becoming a half-fiend (http://www.d20pfsrd.com/bestiary/monster-listings/templates/half-fiend) into Int. It won't matter after you become a demon, if you decide to carry this out to the bitter end, but it will help you get there.
The Fourth Ritual
Fourth Ritual: For many, gaining the half-fiend template is enough, but for those who wish to go even further, a fourth ritual exists. The mortal’s demon lord grants no vision or advice to begin this ritual—the mortal must take it upon herself to honor her demon lord in a manner appropriate to that demon lord’s interests and areas of concern. Once every year, on the anniversary of the day the mortal completed the third ritual, she must perform a special ceremony that recounts her accomplishments over the past year and culminates in a significant sacrifice (usually consisting of the sacrifice of an important member of an enemy faith, or of a lawful or good outsider; in either case, the sacrifice must be at least CR 9). The mortal then makes a DC 40 Knowledge (religion) check, with a cumulative +1 bonus for each previous such ritual she’s performed over the past several years. Success results in a final transformation into a full-fledged demon—the mortal loses all benefits of her previous race or the half-fiend template, but can immediately apply all of her class levels to her new demonic race (for example, a human fighter 10 could become a vrock fighter 10). The type of demon that the mortal transforms into depends on both the nature of her chosen demon lord and the GM’s discretion, but should generally not result in a total number of Hit Dice that more than doubles the mortal’s original Hit Dice.
Congratulations for making it this far. To complete your transformation, you gotta wait a year and then make an appropriate sacrifice. If you preform it well enough, you can leave mortality behind. If not, there is always next year.
Now, exactly how this functions is based on your reading of it, post ritual of course. My reading goes as follows.
Drop your racial traits and the benefits of the half-fiend template and then add pretty much everything the demon has short of equipment to you. However, you can interpret it such that you drop one and keep the other. If you follow this path, keep in mind if you maintain your racial traits, you should treat any discrepancies (such as type) to take after the Demon you become. For example; An elven balor would most likely be Large Outsider (Chaotic, Demon, Elf, Evil, Extraplanar). Your ability scores will receive bonuses based on the demon type (-10 to all even ability scores, -11 to all odd ability scores. Add these as bonuses to what your ability scores are after you take off your former race's traits and the template.)
The Demons
Now, the important question is, after all that work what demon do you become? Well, it does hinge on a few things. Your HD upon the completion of the fourth ritual, the demon you serve, and potentially the things you have done along the way. To make things easier, here is a table containing all the demons I could find, organized by HD, along with the modifiers they would offer. Remember, you cannot become a type of demon that would more than double your HD by becoming one. In essence, you generally cannot become a demon that has more HD than you do at the completion of the fourth ritual.
Do take this list with a grain of salt, as some entries may be absent (especially with the addition of new demons and existence of 3rd party content.)
HD Type of Demon Ability Score Modifiers
2 Dretch (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/dretch)+2 Str, +2 Con, -6 Int
3 Quasit (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/quasit)-2 Str, +4 Dex, +2 Wis
4 Vermlek (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vermlek)+4 Str, -2 Dex, +4 Con, +2 Int, +2 Wis, +2 Cha
5 Abrikandilu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-abrikandilu)+4 Str, +2 Con, -4 Int, +2 Cha
5 Hala (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-hala)+6 Str, +8 Dex, +4 Con, +2 Int, +4 Cha
5 Schir (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-schir)+6 Str, +4 Dex, +4 Con, -2 Int, -6 Wis, -4 Cha
5 Swaithe (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-swaithe)+6 Dex, +4 Con, +2 Int, +2 Cha
6 Brimorak (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-brimorak)+6 Str, +6 Dex, +8 Con, +2 Int, +2 Wis, +4 Cha
7 Babau (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/babau)+10 Str, +2 Dex, +10 Con, +4 Int, +2 Wis, +6 Cha
7 Shadow (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-shadow)Str -, +8 Dex, +6 Con, +4 Int, +4 Wis, +8 Cha
8 Incubus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-incubus)+10 Str, +4 Dex, +8 Con, +6 Int, +4 Wis, +10 Cha
8 Succubus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/succubus)+2 Str, +6 Dex, +10 Con, +8 Int, +4 Wis, +16 Cha
9 Nabasu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/nabasu)+12 Str, +6 Dex, +12 Con, +4 Int, +6 Wis, +8 Cha
9 Vrock (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/vrock)+10 Str, +4 Dex, +14 Con, +4 Int, +6 Wis, +6 Cha
10 Hezrou (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/hezrou)+16 Str, +18 Con, +4 Int, +4 Wis, +8 Cha
10 Kalavakus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-kalavakus)+12 Str, +2 Dex, +14 Con, +4 Int, +6 Wis, +6 Cha
11 Derakni (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-derakni)+16 Str, +12 Dex, +12 Con, -2 Int, +6 Wis, +6 Cha
11 Gibrileth (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-gibrileth)+14 Str, +8 Dex, +14 Con, +2 Int, +6 Wis, +6 Cha
11 Kithangian (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-kithangian)+12 Str, +10 Con, -4 Int, +8 Wis, +6 Cha
11 Shachath (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-shachath)+14 Str, +14 Dex, +16 Con, +8 Int, +10 Wis, +10 Cha
11 Xenarth (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-xenarth-ichor-shark)+18 Str, +4 Dex, +16 Con, -4 Int, -4 Cha
12 Glabrezu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/glabrezu)+20 Str, +20 Con, +6 Int, +6 Wis, +10 Cha
12 Omox (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-omox)+16 Str, +14 Dex, +14 Con, +4 Int, +8 Wis, +8 Cha
14 Nalfeshnee (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/nalfeshnee)+22 Str, +2 Dex, +18 Con, +12 Int, +12 Wis, +10 Cha
14 Oolioddroo (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-oolioddroo)+4 Str, +14 Dex, +14 Con, +10 Int, +14 Wis, +12 Cha
14 Painajai (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-painajai)+16 Str, +6 Dex, +18 Con, +8 Int, +8 Wis, +8 Cha
15 Seraptis (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-seraptis)+12 Str, +12 Dex, +18 Con, +6 Int, +8 Wis, +10 Cha
16 Coloxus (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-coloxus)+8 Str, +18 Dex, +10 Con, +6 Int, +8 Wis, +12 Cha
16 Marilith (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/marilith)+14 Str, +8 Dex, +22 Con, +8 Int, +8 Wis, +14 Cha
17 Lilitu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-lilitu)+16 Str, +10 Dex, +20 Con, +10 Int, +12 Wis, +16 Cha
17 Shemhazian (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-shemhazian)+24 Str, +8 Dex, +18 Con, +16 Wis, +6 Cha
18 Vavakia (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vavakia)+22 Str, +4 Dex, +22 Con, +8 Int, +10 Wis, +12 Cha
18 Vilsteth (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vilsteth)+12 Str, +8 Dex, +14 Con, +12 Int, +6 Wis, +12 Cha
19 Gallu (Warmonger) (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-gallu)+22 Str, +8 Dex, +24 Con, +8 Int, +12 Wis, +14 Cha
19 Vrolikai (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/demon-vrolikai)+20 Str, +12 Dex, +24 Con, +12 Int, +12 Wis, +16 Cha
20 Balor (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/balor)+24 Str, +14 Dex, +26 Con, +14 Int, +14 Wis, +16 Cha
The Price of Power
The higher level you are, the greater your options are when it comes to demons. One could argue you should go for the most powerful demon available to you. After all, you have the most to benefit in the moment, but this comes with its own drawbacks. For one, you are effectively spontaneously gaining anywhere from 2 to 20 levels, and although in each set the benefits are fixed, still the more levels you gain the more powerful than the rest of the group you may become. Furthermore your advancement will slowed down. For classes and characters that don't depend upon the advancement of particular abilities have the most to gain. They won't fall behind in that area as easily, and their might is easily added to your own. For spellcasters, those that have the easiest time becoming demons, they will find that their slowed advancement for being several levels ahead may cripple them in the long run, pending upon how much longer the campaign continues. There is nothing wrong with seeking to become a balor or the 3rd party choronzon, but don't forget there are benefits even for becoming the lowly quasit.
But don't become a dretch. Nobody likes dretches.
Classes that Shouldn't
Unfortunately, becoming a demon is not for everyone. There are some classes that should be addressed when considering undertaking the process.
Druid: While against the natural order of things, druids are among the more common groups that have contention with demons to worship demons or even consider becoming them. Regretfully, given the nature of the process you will be an ex-druid for some time if you go down this path. If after you manage to balance yourself between law and chaos, or good and evil, you may then return to being a druid.
Monk: Most monks require discipline to continue advancing through their training. So unless you are simply a martial artist, keep in mind the fact that once you start down the road to demon-hood, your training will have to be put on hold indefinitely. Fortunately, you retain your abilities.
Paladin: Paladins are directly opposed to demon-kind. If you want to become one, you must stop being a paladin. I advise considering becoming an anti-paladin, so at least your training doesn't go to waste. If you intend to return to being a paladin afterwards, or wish you could retain your paladin abilities while you follow this path, this is not the guide for you. The road to damnation is swift, and the road to redemption once there can be impossibly steep.