What levels of spells would you need to make monsters?

[The Jack]

So, I don't like the idea that some things are arcane creations, but the player can't make them because they're for NPCs to make. Don't ask about ecology the DM just put things here. If they had a wizard do it, why can't my wizard do it. I can understand a discrepancy between clerics, but nothing really should stop the man of oz.
But I think it could be really fun to play a game where a player wants to be the monster master in a dungeon hideout. Cooperative dungeon keeper with the DM playing the attackers? Why not?

Off the top of my head, these things're missing, and could do with levels.

Animate Larger, non humanoid undead. (Warhorse, minotaur, undead beholder...)
Flameskull.
Cawling claw

Animated Armour (note- the "animate object" spell doesn't do this)
Flying sword
Helmed horror
Shield Guardian
Scarecrow.
Gorgon

Chimera
Owlbear
Bullette
Whatever else was the result of "a wizard did it".

Also there doesn't seem to be an option to conjure NE fiends like there is Devils and demons, but it's not like there's a satisfactory system for conjuring outsiders anyway. Planar ally is vague and could give you an imp or a solar depending on the dm, everything else is very temporary. I'd be happier to spend expensive ingredients (or higher spell slots) for more permanent creations/summons.

Now I've probably left stuff out, and there's plenty of stuff people just put into dungeons, but what levels would you make the spells and rituals to make the above?

[DatBoi_1997]

I've thought about giving something similar to this to my players. My biggest suggestion for you when it comes to something like this is to talk to your GM and try to work together to craft some homebrew spells.

In the world I created, something powerful like conjuring up monsters and demons takes either a very powerful wizard and/or multiple wizards. So you might find a level 6 wizard who knows how to cast animate dead, but she wont have anything more powerful than a group of zombies at her disposal, or you might have a group of wizards who managed to animate the skeleton of a Minotaur. Meanwhile an evil arch-mage who managed to acquire the Hand of Vecna might be able to reach into another demention and pull out something more terrifying than anything my players have faced in the whole campaign.

As for homebrew advice, I would suggest looking up the Unearthed Arcana "That Old Black Magic"

This should help you get started with a good concept of what kind of things you might be able to summon and what level they would cost. I would also take note that the "animate dead" spell only allows you to control the monsters for so long. The UA "That Old Black Magic" doesn't actually allow you to control the creatures at all. I would keep that spirit in mind when crafting homebrew spells.

Your last option, and honestly the best thing you could do, if you enjoy playing mages that can create monsters and you like making dungeons, then just DM already. Then you can make all the evil wizards and dungeons you want.
Best of luck to ya.

[The Jack]

But I am the DM. This was an offering to players.


I mean, I'd love the option given to myself, but that's only somewhat relevant.

[Emay Ecks]

I would look at and use the current summoning spells as a template (Conjure Animals, Conjure Celestial, and Conjure Elemental, Conjure Fey, Conjure Minor Elementals, and Conjure Woodland Beings).

Conjure Animals (3rd level) - Summon one beast of CR 2 or lower for 1 hour. Increasing the spell level by 2 lets you summon additional creatures.
Conjure Elemental (5th level) - After a one minute cast time, summon an elemental of CR 5 or lower for 1 hour. If cast at a spell level above 5, increase CR to match spell level (so an 8th level cast would let you summon a CR 8 elemental)
Conjure Minor Elementals (4th level) - After a one minute cast time, summon an elemental of CR 2 or lower for 1 hour. Increasing the spell level by 2 lets you summon additional creatures.
Conjure Woodland Beings (4th level) - Identical to above, except fey instead of elemental.
Conjure Fey (6th level) - After a one minute cast time, summon a fey creature of CR 6 or lower for 1 hour. If cast at a spell level above 5, increase CR to match spell level.
Conjure Celestial (7th level) - After a one minute cast time, summon a celestial of CR 4 or lower for 1 hour. If cast at 9th level, increase the CR to 5.

Xanathar's adds: Infernal Calling (5th level) - After a minute summon a CR 6 or lower devil for 1 hour. Unlike the above spells, the creature is unfriendly to you. If cast at a spell level above 5, increase CR to match spell level + 1.
Summon Greater Demon (4th level) - Summon a CR 5 demon for one hour. It is basically hostile to you, but targets the nearest thing first. If cast at a spell level above 5, increase CR to match spell level + 2.
Summon Lesser Demons (3rd level) - Sumon a CR 1 demon (or worse) for one hour. It is hostile to you.

What I'm noticing is the following:
-Summoning and creature management cannot be done below 3rd level spells (5th level character), and doesn't really get efficient until you use 5th level spells (9th level characters).
-The CR of what is summoned is usually roughly equal to the spell level used once you get past 5th level spells. You can get stronger creatures out by making them hostile to you (basically let them target anything).

If I was the DM I would probably do one of the following:

-Give the above spells a clause of "If this spell is cast every day inside a magic circle (3rd level spell) for one month, and the same creature is summoned each cast, the casting becomes permanent and no longer requires concentration"

-Through mad experimentation using magic, you may use a system similar to the magic item crafting rules (either those from the DMG or whatever is used at the table) to create a creature whose CR is equal to or lower than your highest spell level. This is a time consuming and expensive process with it taking roughly 3 days and 300 gold (these are numbers I made up on the spot, please adjust to match your setting) per CR of the creature. Additional conditions may be required (requires a base creature, requires a certain magical item or location to be present, must be done underwater during stormy weather, whatever) for particular creatures. Some creatures are so difficult to create, that you must have found or been taught the recipe/ritual to make them.

-After capturing a live creature, you may brainwash it to loyally guarding your dungeon. This requires daily casts of Geas (5th level or higher), Dominate Monster (5th level or higher), or Charm Monster (4th level) if I'm feeling generous. The creature can be restrained by maintaining a magic circle (3rd level) around it. This entire process takes 30 - (your spell save DC) days. After that point, the creature is permanently mentally altered into doing your bidding in the best way it can comprehend.

[JackPhoenix]

True Polymorph. If it has stats and fits the CR requirements, you can make it.

[Unoriginal]

Now I've probably left stuff out, and there's plenty of stuff people just put into dungeons, but what levels would you make the spells and rituals to make the above?

All those things don't require spells with levels, they require rituals.

If you know the ritual and have everything you need to perform it, you can perform it, no spell slot needed.

I'm kinda working on a guide for the subject.

Basically, it works like a magic item formula.

Though note that some monsters, like Chimeras, do reproduce naturally. They've just initially been twisted by demons (in the case of the Chimera, I mean).

[Thrudd]

I would make it something to be attained through special research for high level characters. They would definitely need to have built a specialized magical laboratory stocked with equipment and materials for the purpose (costing some tens or hundreds of thousands of gold, or being the object of a difficult adventure). Then spend a specified amount of downtime (probably a week minimum) doing experiments and expend some amount of gold, at the end of which they should make an arcana roll. You'll need to decide how many such rolls (and how many weeks) will need to be passed and at what difficulty in order to get the desired result. Having access to a certain level of spell slots (I'd say seventh at least) and also specific spells could be prerequisites depending on the type of thing being created. You also might require harvesting special materials that will require adventuring, like the bodies of a number of special animals or monsters, a certain type of gem, elements from magical places or another plane, etc.

"A wizard did it" does not mean they did it easily or quickly. Anything that isn't described as a character ability in the book should be something difficult and rare. To create a new form of life, or create anything new, should be a subject of great effort on the part of the character.