So i have been thinking about this spell specifically for a warlock, this is because i noticed not many spells actually consider the unique system that a warlock has.

I made this spell with the idea in mind to limit action economy for the warlock that might happen with create undead and remove some of the complicated requirements of recasting that comes with it.

you can find the spell here https://www.dndbeyond.com/spells/1712001-undead-companion.

with that in mind this is the very first version of the spell and i am sure it's far from being balanced so i would like to hear your thoughts about it.

Issues i am aware of:

• it will add some confusion between DM and the warlock if the DM decides to cut down the states of the creature too much that it makes it practically useless.
• the spell has no mention of skills or magic spells, which i am still conflicted about, i am thinking of just making all of them disappear but i would love to hear if someone has any ideas about how to keep some without making the creatures overpowered.
• the new version i am working on makes it explicit that no legendary or lair actions are retained.
• the missing information part is to give the DM the chance to remove essential information if the warlock decides to use the spell on a key character, however at lvl 20 this will become impossible, as i have never played a character to lvl 20 before i am not sure if that's balanced, keep in mind that this is not an everyday use spell, it's casting time alone is enough to deter many from spamming it on every character and sacrifice a good combat creature for some extra information.

Please let me know if you notice some other issues with it as this is my first homebrew spell i am admittedly not that great at it.

Edit:

Apparently i forgot to include that the CR/level of the reanimated creature should not be higher than the spell level.

Uhh… a few things:

1. You have it listed as a 6th-level spell, but then have an “At Higher Levels” section and it’s only for an Undead Warlock. Warlocks cannot Upcast their Mystic Arcana like that, they can only ever cast those spells at their base level. The rules for upcasting explicitly require using spell slots to upcast, and Mystic Arcana are cast without using spell slots. That right there is the thing that jumped out at me the most.
2. You put absolutely no restrictions whatsoever on which creatures can be reanimated.
   • That’s waa—aay too powerful for a spell of that level. Compare it to the 6th-level spell create undead, it lets a Warlock create 3 CR1 ghouls at the same level.
   • If you limited it to a “Large or smaller Beast or Humanoid” with a CR cap of, say, 5, then you can skip all of the stuff about magic items, Legendary/Lair/Mythic actions, etc. Creatures that fit those criteria don’t have any of that stuff.
3. There are few things DMs dislike more than the phrase “the GM has the creature's game statistics.” Just an FYI. That’s why newer spells like summon undead include the game statistics for the summons in the spell itself.

All in all, I think you would do better making a 6th-level adaptation of the summon undead spell with a duration of “Until Dispelled,” and include your rider about how if you cast it again the old one tries to kill you. (That part was good.) I hope that helps.

For the first point some DMs do allow a warlock to choose a lower level spell for their higher level mystic Arcana i have done some research but in general you are right and i think it's better to stick to the rules so i will remove that part.

for the second point, that's my fault i noticed too late that i didn't include that the CR is capped at the spell level which is 6, i did have that in mind otherwise u will get an ancient dragon with a level 6 spell which is OP even with it's stats halved.

coming to the point of capping the CR Do you think halving the stats of a CR 6 is still OP for a sixth level spell i kinda went over some of the CR 6 creatures and if you have their stats they are nearly on the same level as 3 ghouls (some slightly stronger, some slightly weaker) but i am not an expert on monsters so i am not sure there is no 6CR monster that is not OP even with stats halved.

for the third point i 100% agree, the look my DM gave me when i showed this to him was priceless xD

The goal of the spell is to give some more flavor and options to a class that is otherwise IMHO very reliant on EB i noticed that warlocks turns tend to be very short especially if the fight goes for more than 2-3 rounds they have very limited bonus actions and they basically just EB their way through the rest of the fights.

My DM suggested adding another mechanic to limit the times this can be casted to help mitigate the extra work the DM has to put into this, i am not sure what kind of mechanic that would be though.

I still think like you said DM's would feel burdened by this since they will have to decide on the stats which might leave the warlock dissatisfied or make the party a one man act.

Do you think limiting the CR to 5 and then giving some rules to determine the stats would make it easier for DM's with a sentence at the end DM hold's the right to nerf stats if he thinks they are overpowered, because like my DM said the very existence of this spell would make the DM think twice about what creatures the party encounters out of fear that he would give an overpowered creature to the warlock.

I totally understand this spell is very unconventional and most summon spells would give a precise stat block to avoid balance issues and rightly so, but this is my humble attempt at making a spell that i know would make it much more interesting and investing for a player to create his own type of creature.

If you use the convoluted system the DMG lays out for how to build combat encounters, 3 CR1 ghouls are to be considered just a smidge more challenging than a single CR4 banshee or ghost. I was being generous when I suggested CR5 as the cap. And remember, create undead has all those restrictions on it that you don’t want your spell to have.

Why not just include an “undead template” in the spell’s description that could be applied to any CR3 beast or humanoid. That would probably be more balanced and still in line with what you are going for.