How to make a weapon that grows as the weilder grows?

[Rfkannen]

So I was thinking about the sword from 1/2 prince and the dagger from Dungeon ni Deai o Motomeru no wa Machigatte Iru Darou ka and how they might be cool in a d&d game. The basic principle of both weapons is that as the wielder becomes more powerful the weapon does as well, slowly evolving into a more powerful form becomeing a great weapon. I also think that pathfinder unchained has something like that, but I do not own that book.

But anyway, I was wondering if anyone thought that such a weapon could work in 5e, and if so, how?

[pibby]

Sure, why not? It's the same thing as exchanging a lesser magic weapon for a better one during a campaign.

I guess if I was making a growing sword it would first start out as a magic weapon with some feature on it relevent to the campaign. Then somewhere between plot points the weapon would evolve and gain new features that help further the plot.

But that's just a general statement. What exactly do you plan on using the sword for?

[Rfkannen]

Sure, why not? It's the same thing as exchanging a lesser magic weapon for a better one during a campaign.

I guess if I was making a growing sword it would first start out as a magic weapon with some feature on it relevent to the campaign. Then somewhere between plot points the weapon would evolve and gain new features that help further the plot.

But that's just a general statement. What exactly do you plan on using the sword for?

yeah my problem is figuring out exactly when it upgrades or how. All the actual mechanics. This post was about thinking about how to make an evolving weapon in general. Like mechanically how to keep it balanced, when to give it features and all that.


Personally? I have been thinking about starting up a game based off a cross between one piece and adventure time. Where the players are looking for something or other (not sure what) and going from island to island with zany characters and dungeons. Not sure what they would be looking for or why, but thats a generall idea I had. If you have any ideas for an evolving weapon for such a game I would love to hear 'em.

ps. If anyone has any ideas and would be willing to whip up an example I would be eternally greatful.

[TurboGhast]

Make the item gain abilities as the weilder levels up, say...

At Lv5 becomes +1;
At Lv8 charging with it adds 5 ft to your speed;
At Lv10 becomes +2.
Ect.

3rd and 4th edition are much more conductive to this idea since they assume you have magic items, and are expected to be upgrades periodically, so that you can keep up.

[calebrus]

If you're going to do something like that, the best way that I've found is to have one for every member of the party. Ancient, powerful relics, that are needed to defeat some evil.
They all grow in power when in proximity to each other, so the more of them that the party finds, the more powerful they become. And no one feels like they got left out wile someone/everyone else gets cool shiny stuff, because theirs will come eventually.
And even better, you don't have to *plan* out when to increase their power. When you think an increase is warranted, the party finds another piece of the set.

If you do that, just make sure to hand out a few other things here and there to get the players by until their real stuff comes along (but not good stuff, or they might not want their real stuff when it comes).

[Easy_Lee]

That depends entirely on how common, and how powerful, other magic items are in your campaign. If the strongest any other weapon can get is +1, then it would make sense for the weapon to start as magical and become +1 at some point. If they go up to +3, like in many campaigns, then I would say use the wielder's proficiency divided by 2, rounded down, starting at level 5. The table would look like this:


LevelBonus
1Magic
2Magic
3Magic
4Magic
5+1
6+1
7+1
8+1
9+2
10+2
11+2
12+2
13+2
14+2
15+2
16+2
17+3
18+3
19+3
20+3

Additionally, I think that you should give the weapon some unique, unchanging bonus to give it personality. Two good examples are the Monkey King's staff from mythology, which could lengthen at will, and Kozah's Needle from the Drizzt novels, which could absorb and redirect lightning-based attacks as well as building up a charge.

[Fralex]

The basic principle of both weapons is that as the wielder becomes more powerful the weapon does as well, slowly evolving into a more powerful form becomeing a great weapon.

You actually gave me an idea for a magic weapon just now. Interpreting what you said a little differently, what if there was a weapon that got stronger in response to your temporary increases in power? Like, a bonus when you make an attack using it with advantage, or a bonus for every non-detrimental spell currently affecting you (sorta like Magic the Gathering's heroic mechanic)?

[Easy_Lee]

You actually gave me an idea for a magic weapon just now. Interpreting what you said a little differently, what if there was a weapon that got stronger in response to your temporary increases in power? Like, a bonus when you make an attack using it with advantage, or a bonus for every non-detrimental spell currently affecting you (sorta like Magic the Gathering's heroic mechanic)?

That's interesting. What if, if you just cast a spell, have an active spell affecting you, or are concentrating on a spell, your weapon does additional damage equal to the spell's power? That'd be pretty simple and easy to implement, so long as it only queued off of the most powerful effect at a given time. Could be really nice for a bard.

[pibby]

yeah my problem is figuring out exactly when it upgrades or how. All the actual mechanics. This post was about thinking about how to make an evolving weapon in general. Like mechanically how to keep it balanced, when to give it features and all that.


Personally? I have been thinking about starting up a game based off a cross between one piece and adventure time. Where the players are looking for something or other (not sure what) and going from island to island with zany characters and dungeons. Not sure what they would be looking for or why, but thats a generall idea I had. If you have any ideas for an evolving weapon for such a game I would love to hear 'em.

ps. If anyone has any ideas and would be willing to whip up an example I would be eternally greatful.

If it's balence you're looking for check the DMG for expected starting equipment for the various levels of fantasy. That might give you a good start.

Like I've said before, if I was the player I'd like it if every time the magic weapon leveled up at the end of session I could suggest the new properties of that weapon based on what's going on in game. I'd then let the DM decide exactly how that would manifest.

[coredump]

I think part of the fun in the game is getting new stuff, earning improvements.

To that end, I would not make the 'growth' automatic, I would key it off of something that needs to be achieved.

1) Perhaps it has several parts that must be found and put together. A staff could be in sections, the more sections you find, the more powers it has. A sword may need to find the pommel gem, and the scabbard. etc.

2) Perhaps doing something will allow the weapon to grow. Killing a black dragon, or one of each color Slaad. Or you need to leave it overnight in a pool consecrated to an ancient God no longer worshiped (So now you have to find an ancient temple)

Basically, use it as adventure hooks.

[PeterM]

I've been thinking for awhile that if I ever run a game (hey, it could happen) I'd allow something like this. I think I'd introduce a fairly simple magical ritual that transfers magical effects from one object to another similar object, with a fairly broad definition of "similar." So if you want to use your family's ancestral guisarme-voulge but find a lousy +1 longsword in a treasure room, you can empower your guisarme-voulge fairly easily and toss the now useless longsword. You could turn your family's signet ring into a ring of protection the same way.

[Giant2005]

I've been thinking for awhile that if I ever run a game (hey, it could happen) I'd allow something like this. I think I'd introduce a fairly simple magical ritual that transfers magical effects from one object to another similar object, with a fairly broad definition of "similar." So if you want to use your family's ancestral guisarme-voulge but find a lousy +1 longsword in a treasure room, you can empower your guisarme-voulge fairly easily and toss the now useless longsword. You could turn your family's signet ring into a ring of protection the same way.

What if he absorbed the power of that +1 Longsword and then absorbed the power of a Flame Tongue Longsword? Would it be a +2D6 fire damage weapon or would it be a +1, +2D6 fire damage weapon?
The latter sounds really awesome for a villain that was out to try and absorb all of the magic from the world.

[Gritmonger]

You could start with an intelligent weapon that simply refuses to allow some powers to be used until it thinks the wielder is ready or worthy. I think there was a version of artifacts that used a similar mechanic - depending on what you did that aligned with the item's purpose, you gained or lost access to major, minor, and trivial bonuses until at the high end it was effectively an artifact, while at the low end it was almost worthless and then eventually found a way to leave you.

[Safety Sword]

This is actually a great way to add adventure hooks and get martial characters to care about research in your world.

The sword may have a prophecy attached to it that describes actions the wielder has taken (or will take) to confirm they are worth of the weapon. Player will then generally look for situations in game to let their characters fulfil the prophecy and that leads to all sorts of fun.

The half-orc barbarian in my game is currently trying to make contact with a celestial being to help him on a quest to be worthy of the sword he has found. Just make sure that simple identify spells don't ruin the whole thing.

Identify tells you all the properties of the weapon, but only at the time that you cast it ;)

[MadGrady]

While somewhat arbitrary, you could use the Rarity tables and suggested character level to space out the enhancements.

Starts off with an Uncommon weapon - minor property like a +1 Sword

At level 5 it becomes a Rare Item (+2 with another benefit)

At level 11 it becomes Very Rare (+3 with a few benefits)

and at level 17 it becomes Legendary (Artifact level)


You could add smaller benefits in between the levels so that it isn't such a grind between upgrades.

[PeterM]

What if he absorbed the power of that +1 Longsword and then absorbed the power of a Flame Tongue Longsword? Would it be a +2D6 fire damage weapon or would it be a +1, +2D6 fire damage weapon?
The latter sounds really awesome for a villain that was out to try and absorb all of the magic from the world.

I'd say PCs and the vast majority of NPCs would only be able to attach one effect to any given object. So if you want the +2D6 fire damage, you would destroy the +1 bonus when the newer bonus is applied. Or you could cast the ritual twice and switch the +1 bonus to a third weapon that could be used or sold, then add the +2D6 fire to your original weapon.

I do like the idea of a Big Bad who's figured out how to combine magical effects, I have to admit. That could get out of hand real fast, though.

[Mellack]

This is actually a great way to add adventure hooks and get martial characters to care about research in your world.

The sword may have a prophecy attached to it that describes actions the wielder has taken (or will take) to confirm they are worth of the weapon. Player will then generally look for situations in game to let their characters fulfil the prophecy and that leads to all sorts of fun.

The half-orc barbarian in my game is currently trying to make contact with a celestial being to help him on a quest to be worthy of the sword he has found. Just make sure that simple identify spells don't ruin the whole thing.

Identify tells you all the properties of the weapon, but only at the time that you cast it ;)

I believe Pathfinder had something along those lines for artifacts called concordance. Basically the more things you did that aligned with the goals of the item, the more power it would let manifest. If you did things counter to it's goals, it would pull them back or even penalize you in some manner.

[Safety Sword]

I believe Pathfinder had something along those lines for artifacts called concordance. Basically the more things you did that aligned with the goals of the item, the more power it would let manifest. If you did things counter to it's goals, it would pull them back or even penalize you in some manner.

That would make more sense if it were an intelligent weapon that had it's own personality and goals.

There were these kind of weapons in D&D 3.5 (and I can't recall their actual designation at the moment...) but they were poor because they actually gave you penalties as well as bonuses. The basic idea was that you had some sort of ancestral weapon that you needed to spend time and resources on to unlock powers at various levels.

Whilst I like the concept, the implementation was poor in that edition of D&D.

[Chadamantium]

Weapons of Legacy from 3.5.

I really enjoyed how they did it in there even if some of the weapons turned out to be not so powerful.

I don't have the book on me but there was a leveling table that went with your character level that gave the weapon bonuses as you level. at certain points there would be some peered I it so prerequisites you would have to do to achieve the next tier of weapon. I'm sure someone could come up with balanced table that could be applied to make a legacy weapon.

[Safety Sword]

Weapons of Legacy from 3.5.

I really enjoyed how they did it in there even if some of the weapons turned out to be not so powerful.

I don't have the book on me but there was a leveling table that went with your character level that gave the weapon bonuses as you level. at certain points there would be some peered I it so prerequisites you would have to do to achieve the next tier of weapon. I'm sure someone could come up with balanced table that could be applied to make a legacy weapon.

That's them. I think you'd be better off making each weapon unique rather than a catch all table. These kinds of weapons should be personal.

[CyborgGold]

I just spent a little while creating a weapon scheme that does just this. I would love some feedback on my idea, hopefully it isn't over or under powered.

The weapons: These weapons are sensitive to the ancient gods. In addition to growing stronger by the charts listed below they can also be blessed by the ancient gods themselves. By bringing them to long forgotten temples and bathing them in the enchanted pools there, the weapons gain the favor of the ancient gods and gain new abilities. The god's each have a ritual that needs to be completed to gain their favor. The player attuned to the weapon must face the challenge alone overnight. Each weapon (or set if multiple weapons exist to be used together) can hold a number of favors based on the below chart. If these weapons use favor points they gain three favor points when favor is granted. If the weapon loses all of their favor points, the favor is lost as well.


Level:
1 - +0
2 - +0
3 - +0
4 - +0
5 - +1, can now hold one favor
6 - +1
7 - +1
8 - +1
9 - +2, can now hold two favors
10 - +2
11 - +2
12 - +2
13 - +2
14 - +2
15 - +2
16 - +2
17 - +3, can now hold three favors
18 - +3
19 - +3
20 - +3


Favor of the Nine: Symbol; Roman Numeral 9

The Nine are a group of legendary warriors blessed by the gods themselves. Favor points can be spent once per day to replace normal damage with 9d4 damage. This ability can only be used before rolling for attack. Weilders of weapons adorned with the favor of the Nine may also cast Aid of the Nine to summon the Nine themselves to aid them in battle. Three full rounds of combat must pass before the Nine arrive. If they are summoned for a task which they deem unworthy, they will refuse to offer aid and the weapon will lose Favor of the Nine. However, if the cause is noble and just they will become allies for the duration of the battle. The aid of the Nine can only be used once, successful or not.


Favor of Auril: Symbol; 6 point snowflake

Auril is the goddess of winter. Favor points can be spent to add an additional 1d6 cold damage per ½ character level, must be declaired before the attack roll. If the weapon is enveloped by fire three times, it loses favor and cannot regain favor.


Favor of Azuth: Symbol; Left hand pointing upwards outlined in fire

Azuth is the god of wizards and magic. Favor points can be spent to increase spell DC by 2 for the duration of the battle or until the weilder is knocked unconscious. The weilder can also re-roll any 1 rolled while this effect is active.


Favor of Bane: Symbol; Upright right hand, thumb and fingers together

Bane is the god of tyrany. Bane's favor allows the weilder to enter a state of rage regardless of class. When raged a player gains advantage on Strength checks and saves; Strength based attacks add a modifier to attack damage equal to ½ their level; player gains resistance to bludgeoning, piercing, and slashing damage; Cannot cast spells while raging. Rage ends if knocked unconscious or if your turn ends and you haven't attacked a hostile creature or taken damage since your last turn. Players can rage twice per long rest as a bonus action. If a player loses rage due to not attacking or receiving damage twice in one battle, the weapon loses favor.


Favor of Beshaba: Symbol; Black Antlers

As goddess of misfortune, Beshaba's favor points can be spent to give one opponent disadvantage on all attack and saving rolls for the duration of the encounter or until they are knocked unconscious.


Favor of Bhaal: Symbol; Skull surrounded by blood droplets

Bhaal is the god of murder. Once per day the favored weapon can attempt a lethal strike at a cost of one favor point. Lethal strike ignores the weapon's standard attack. Player makes an attack roll with advantage. On a roll of 18 or greater the target makes a Wisdom saving throw. If they roll less than 10, the strike hits killing them instantly. If they roll 10 or higher, the strike deals damage equal to 1d10 per half character level. If Lethal strike makes a successful instant kill, it loses favor and cannot regain this favor again.


Favor of Chauntea: Symbol; A blooming rose over grain.

Chauntea is the goddess of agriculture. This favor allows the weilder to cast Hero's feast once without any casting time or cost association. Upon use of Hero's feast the weapon loses favor.


Favor of Cyric: Symbol; White jawless skull on black sunburst

Cyric is the god of lies. This favor grants the weilder advantage on any rolls for deception or investigation checks to see if someone is being decitful. If you fail twice to decieve or percieve deception in one day, this favor is lost.


Favor of Deneir: Symbol; Lit candle above an open eye

Deneir is the god of writing. This favor grants +1 to intelligence, and allows the weapon's weilder to read and write any language at the cost of one favor point.


Favor of Eldath: Symbol; Waterfall plunging into a still pool

As goddess of peace, Eldath's favor points can be spent peacefully end any encounter. If any aggressive actions are taken after this ability resolves, the battle resumes. If the weilder is the one that takes this aggressive action this favor cannot be regained.


Favor of Cond: Symbol; Toothed cog with four spokes

Cond is the god of craft, Cond's favor grants advantage to tinkering checks. If a player fails a tinkering check they can immediately expend one of three favor points to re-roll the check without advantage.


Favor of Helm: Symbol; Staring eye on upright left gauntlet

As god of protection Helm's favor points grant the weilder +2 to AC for 24 hours. If the weilder recieves two critical hits in one day the weapon loses favor.


Favor of Ilmater: Symbol; Hands bound at the wrist with red cord

Ilmater is the god of endurance. This favor allows the owner of the weapon to ignore exhaustion completely. Whenever a player would suffer a point of exhaustion they instead use one of three points of favor.


Favor of Kelemvor: Symbol; Upright skeletal arm holding balanced scales

Kelemvor is the god of death. Kelemvor's favor points allow the weilder to turn any undead into an ally unless they make a DC 13 Wisdom saving throw. If this action fails three times the favor is lost.


Favor of Lathander: Symbol; Road traveling into a sunrise

Lathander is the god of birth and renewal. Lathander's favor allows the weilder to automatically succeed death saving throws. Whenever this ability is triggered, the player is revived with one HP and rises to their feet as if they were being lifted by an invisible force and one favor point is used.


Favor of Leira: Symbol; Upside down triangle containg a swirl of mist

Leira is the goddess of illusion. The weapon's weilder can spend one favor point to create an illusionary copy of themselves once per long rest. This copy is an exact copy of the weilder, only with the following modifiers. The illusion holds weapons that look similar to the weilder's, however they are not magical or enchanted and thus only deal the damage of a basic weapon. The illusion also cannot cast magic, and has abilities that are 2 less than the weilder. The illusion dissipates if it were to take damage.


Favor of Lliira: Symbol; Three six-pointed stars arranged into a triangle

As the goddess of joy, Lliira's favor can be used to make a character extremely happy for 24 hours. While under this effect the character knows you created this happiness and will be willing to offer any assistance you ask unless the target is knocked unconscious. If you ask the target to do something that would harm to it favor is lost and cannot be regained. If the target feels they have been taken advantage of or used the favor is lost and can not be regained.


Favor of Loviatar: Symbol; Nine-tailed barbed flail

Loviatar is the goddess of pain. Player can spend one favor point to add one damage dice to any successful attack dependent on level. This favor can be used after a successful attack roll. Damage dice are allocated as follows:

Level:
1 = 1d4
2 = 1d4
3 = 1d4
4 = 1d4
5 = 1d6
6 = 1d6
7 = 1d6
8 = 1d6
9 = 1d8
10 = 1d8
11 = 1d8
12 = 1d8
13 = 1d10
14 = 1d10
15 = 1d10
16 = 1d10
17 = 2d6
18 = 2d6
19 = 2d6
20 = 2d8


Favor of Malar: Symbol; Clawed paw

As god of the hunt Malar's favor allows the caster to cast Hunter's Mark once per long rest. If the player is not able to catch/kill the marked creature one favor point is lost. Hunter's Mark: Range = 90ft; cast as bonus action; Duration = 1 hour; You choose a creature you can see within range and mystically mark it as your quarry. Until the spell ends, you deal an extra 1d6 damage to the target whenever you hit it with a weapon attack, and you have advantage on any Wisdom (Perception) or Wisdom (Survival) check you make to find it.

Favor of Mask: Symbol; Black mask

As god of thievess Mask's favor grants the weilder an advantage on stealth checks. Upon a failed stealth check the favor loses one favor point.


Favor of Mielikki: Symbol; Unicorn head profile

As goddess of forests, Mielikki's favor grants the weilder favored terrain in forests. While in a favored terrain characters have advantage on all attack rolls, saving rolls, and ability checks and are not slowed by difficult terrain. The weilder may also use a favor point to cast Commune With Nature. If the character causes a forest to become damaged in any way this favor is lost and cannot be regained.


Favor of Milil: Symbol; Five-stringed harp made of leaves

Milil is god of poetry and song. All performance checks made wile weilding a favored weapon have advantage. Inspiration dice are one dice level higher (ie 1d4 becomes 1d6. 1D12 becomes 1d12 + 1d4). Favor points can be spent to automatically pass a failed performance check.


Favor of Oghma: Symbol; Blank Scroll

Oghma is the god of knowledge. Weilder gains +1 to Wisdom. Favor points can be spent to automatically succeed a failed Wisdom saving throw.


Favor of Savras: Symbol; Crystal ball containing many eyes

Savras is the god of divination and fate. Weilder can use one favor point to cast Scry on one creature or location.


Favor of Selune: Symbol; Pair of eyes surrounded by seven stars

Selune is the goddess of the moon. Weilder gains truesight at the cost of one favor point. Can be used as a bonus action during combat.


Favor of Shar: Symbol; Black disc with a gold border

Shar is the goddess of darkness and loss. Weilder can spend one favor point to cast darkness without any component use. Darkness: Range = 60ft; Duration = 10 minutes; takes 1 action to cast; Magical darkness spreads from a point you choose within range to fill a 15 foot radius sphere for the duration. A creature with darkvision cannot see through this darkness and nonmagical light can't illuminate it. If the point you choose is an object you are holding or one that isn't being worn or carried, the darkness emanates from the moject and moves with it. Completely covering the object with an opaque object blocks the darkness. If this spell's area overlaps with an area of light created by a spell the spell that created the light is dispelled.


Favor of Silvanus: Symbol; Oak leaf

Silvanus is the god of wild nature. Favor points can be spent once per long rest to convince any wild creature to become your ally until it is knocked unconscious or 24 hours has passed.


Favor of Sune: Symbol; Face of a butifal red-haired woman

Sune is the goddess of love and beauty. Favor points can be spent to remove up to 10 years from any target's age. Target cannot become younger than the age at which they become an adult. Favor points can also be spent to heal all creatures within 10ft of yourself by 4d8.


Favor of Talona: Symbol; Three teardrops in a triangular shape

Talona is the goddess of disease and poison. Favor points can be spent to afflict all creatures within 10ft of your targeted creature with poison. These creatures take 1d6 poison damage upon casting, and suffer an aditional 1d6 + 1d6 for each turn that has passed since becoming poisoned points of damage at the beginning of each turn. (ie the second turn deals 2d6 poison damage, 3rd turn 3d6, etc.)


Favor of Talos: Symbol; Three lightning bolts radiating from a central point downwards

Talon is the god of storms. Favor points can be spent once per long rest to summon a storm above a point of your choosing with a range of 60 ft. The storm has a radius of 30ft and at the end of each of your turns makes three lightning strikes distributed evenly to creatures within the area of effect. Creatures in the area roll a Dexterity saving throw against a DC of 15. On a successful save no damage is delt. On a failed save the creatures suffer lightning damage equal to 1d10*1/2 weilder's level. Lightning strikes creatures that have not been hit yet first, followed by those who have been hit the least, etc.


Favor of Torm: Symbol; White right gauntlet

Torm is the god of courage and self-sacrifice. Weilders of weapons with this favor have advantage against any saving throws made due to the weilder taking actions that put themselves in harm's way to protect an ally. Favor points can be spent to automatically save against a failed check against fear, confustion, or mind control effects.


Favor of Tymora: Symbol; Face-up coin

Tymora is the goddess of good fortune. Weilder can re-roll any die that lands on a 1. Favor points can be spent to re-roll any failed saving throw.


Favor of Tyr: Symbol; Balanced scales resting on a warhammer

Tyr is the god of justice. Favor points can be spent to force a creature to reveal any actions they have taken in the past for which they have not attoned. The favor point can be regained if an appropriate action is taken to punish the creature for their discressions. If the punishment is too severe to serve as attonment for the discression favor is lost and cannot be regained.


Favor of Umberlee: Symbol; Wave cresting to the left

Umberlee is the goddess of the sea. Weilder gains waterbreathing. Favor points can be spent to turn any water based creature into an ally for up to 24 hours or until the creature is knocked unconscious.


Favor of Waukeen: Symbol; Disorganized stack of gold coins.

Waukeen is the goddess of trade and wealth. Weilder gains favor of any merchants they deal with granting them a 10% discount on goods and merchants pay an extra 10% when you sell them goods. Favor points can be spent to gain knowledge of locations containing lost treasure caches.


So I was thinking about the sword from 1/2 prince and the dagger from Dungeon ni Deai o Motomeru no wa Machigatte Iru Darou ka and how they might be cool in a d&d game. The basic principle of both weapons is that as the wielder becomes more powerful the weapon does as well, slowly evolving into a more powerful form becomeing a great weapon. I also think that pathfinder unchained has something like that, but I do not own that book.

But anyway, I was wondering if anyone thought that such a weapon could work in 5e, and if so, how?

[Arkhios]

I'm kinda fond of the pattern I've used: The magic weapon that grows in power would gain further abilities depending on the character's tier.

1st tier (levels 1 to 4): Magical, but no bonuses. (point being: it can bypass resistance against non-magical weapons, but little else in that regard)
2nd tier (levels 5 to 10): +1 bonus.
3rd tier (levels 11 to 16): +2 bonus.
4th tier (levels 17 to 20): +3 bonus.

Other than that, each tier would obviously add something extra, so that the weapon isn't incredibly boring.

Same could apply to a magical armor as well.

[Arial Black]

There is an example in the DMG: The Moonblade.

This weapon starts off at +1 and then, on the death of the owner gets a new power or new +1 to a maximum of +3.

With the weapon you are creating, instead of rolling on that table (or a similar table you create) whenever a new wielder turns up, roll whenever the DM thinks is appropriate. Kind of like a milestone for the weapon. The DM can use various triggers: character level, achieving plot goals, whenever the DM would give out cool magical stuff the PC gets a weapon improvement instead, for any reason the DM wants.

It works a treat!