Can the True Polymorph spell transform a creature into a custom creature?

Question

The basis for my question is this line in the true polymorph spell (emphasis mine):

Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose c⁠hallenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). The target’s game statistics⁠, including mental ability scores, are replaced by the statistics⁠ of the new form. It retains its alignment and personality.

By RAW, does that mean that you can effectively craft a creature - say, a "PC" with class levels - outside of established Monster Manual entries? (A new form.)

Sidenote: I am aware that a great deal of true polymorph shenanigans are up to DM interpretation. Our server has multiple DMs, so I am seeking evidence-supported answers that we can all fundamentally agree on without straying too much into our own interpretations.

Answer 1

Probably not (except for some minor customization).

As you said, many True Polymorph shenanigans fall under DM fiat. This includes whether you can polymorph into a specific creature (and not a generic example of a specific kind of creature), and others of the sort.

If you turn a creature into another kind of creature

You, the caster, can essentially choose a kind of creature. While the DM has leeway to create new monsters for his world and adventure, players (in every table I've ever been, but again, there can be exceptions) don't usually have the power to just invent a new kind of monster.

I would adjudicate, for your server, that you can only True Polymorph a target into one of three:

• an officially existing creature, taken from the MM, VGM, MToF, etc. This includes variant options of monsters (like the Shadow Dragon). All of these should still have the approval of the DM, who may claim a given creature does not exist in his setting

• an officially existing NPC, with a specific (DM approved) race. Appendix B of the MM and of VGM describe class-based NPCs (Bards, Warlocks, etc) as any race, and claims the DM can can add racial traits to further customize them. it seems logical to assume that, when True Polymorphing into one, the DM (or player) could logically pick an adequate race.

• a homebrewed creature that the DM has created and that the players know exists in the setting

This basically prevents players from being able to make up some creature that fits exactly all the requirements they need for some specific problem, it gives the DM a basis for obtaining statistics for creatures (either from the official sources or from their own notes), and it also gives the players a fixed set of sources (the books and the DM's notes) from which they can draw ideas and kinds of monsters.

Being a server, this should be a proposition that follows your social contract (e.g., all DMs agree on it, or something similar).

Answer 2

No, you as a player cannot craft a new creature, you may only choose a creature from a source book, or that your DM has created, so long as such creature has a challenge rating.

In the Dungeon Master's Guide, there is a section called 'Dungeon Master's Workshop' which contains a section on creating monsters; so custom monsters are RAW, as long as the DM is the one that created them.

Since your server features multiple DM's, it would be best that they either A- collaborate on the creature (if there is no swapping of players and DM's), or B- adjudicate the creature for its holding to the creation standards (if players alternate being DM).

As to morphing a creature into a 'PC' (or NPC if it isnt to be played by a player) with class levels, no.

In the text it says:

any kind you choose whose c⁠hallenge rating is equal to or less than the target’s

This tells us that the focus is upon creatures with a challenge rating.

The following text:

(or its level, if the target doesn’t have a Challenge rating)

is in reference to the number on the original form (CR or level) being important, not whether the new form can have class levels.

So to summerize:
True Polymorph can be used to turn any creature into any other creature, as long as the new form has a challenge rating number equal to or lower than the rating or level of the original form, and as long as the new form is one that was chosen by the caster, but originated from published materials or the DM(s).

Answer 3

Basically yes. So long as its technically under your level in CR rating

(so under 17 - 20 perhaps higher if you are using epic levels)

The only thing I would say needs to be the case is. That it needs to be able to exist.

• CR 13 Vampire with mage levels (so long as its under your rating your fine)
• Level 17 - 20 Fighter (Fine)
• Mindflayer Warlock (probably not for two reasons 1 you turning into a Mindflayer that's going to mess with you, but the second part is impossible without already having a patron lined up)
• Adult Dragon with Paladin level? (Go ahead It sounds awesome)

However, The following would be impossible at least out of hand.

• A completely new creature that never existed (you would need to be able to connive of it)
• Something perfectly CR rating legal and GM excepted (Still no. Pitchforks down people let me finish)

Basically if they want to make up a completely new creature you should let them do it... IC do it that is. After all that is how we got the Owlbear, and a few other creatures. Have them go through the process of creating this new creature in the world first. After that than they can True Polymorph into it.

Could you just GM fiat it?

of course you could, but tell me which is more fun the Wizard doing some world shaking stuff creating an entirely new creature that will populate the world. Journeying to places far in wind to get the essence's required to make it. A whiff of celestial, a touch of wind elemental, and throw in a fox for good measure. depending on how complex they want to make it. The wizard and the party could have all this stuff done before lunch (planeshift + teleport), or by the end of the week. Besides it might give the party some more ideas to add to the creature. Hell the rest of the party might get in on it as well.

Seriously though its 9th level magic you got to accept the OP at this point.