Hey all, I just got this homebrew published for my Great Old One, Pact of the Chain Warlock:

https://www.dndbeyond.com/monsters/557088-great-old-one-familiar

I made it because it seems like most Pact of the Chain Warlocks take the Imp as their familiar because it's so powerful, but it REALLY doesn't make any sense why a Great Old One would have a devil as their familiar. For my GOO Chainlock I really wanted something that matched the Lovecraftian theme of the character, and I wanted something I could show the other players so they get a sense of it. 

I left all the stats and abilities basically exactly the same to assuage any fears that I was trying to give my familiar more powers. The main things changed was just the type, which I switched from "Devil" to "Aberration" (so it's in the same category as Mind Flayer and Beholder, which I find appropriate) and setting the alignment to Neutral (IMHO that's what most the GOO are, but really it doesn't matter what it says on the creature sheet!)

Personally I'd like to make some changes to it for the sake of RP, like altering the animals it can shapechange to (but keeping them within the simple animals list for normal familiars) and altering it's attack to be psychic damage or something like that, but I figured leaving it alone was safer from a balance perspective. 

Seems fine, as a DM i would always try to steer GOOlocks away from reskinning the Imp for their familiar....could I interest you in an Oblex Spawn? (page 218 of Mordenkainens Tome of Foes) or a Gazer? (page 126 of Volo's Guide to Monsters)

I checked out Gazer and honestly it's stats just didn't compare. I don't want to be a min-max'er, but the Imp really stands out in terms of utility and survivability with it's invisibility, hands, and strength. From what I saw of Gazer and Oblex Spawn (having trouble getting good info about that one) I'd be giving up a lot to take them, and the Chainlock is already giving up a lot.

(Also of course I can't see those creatures here on D&DB because I only have the PHB, I think by making this re-skin it can be useful for people with just the SRD access who want a nice RP-friendly option)

As boring as it is for most Chainlocks to take an Imp because it's the best choice, it's also a reality and as a first time Warlock I am hearing that siren call :P

In that situation, a re-skin seems at least better than flat out just using the basic Imp.

As a DM, what would you change about the Imp stats for this creature to make it more GOOey?

But you do make a good point about increasing the list of options! A creative DM could offer similarly-capable creatures outside the imp/pseudragon/quasit/sprite list and just make the game balance out in the end (and maybe avoid the downsides of having an invisible infinite-range scout which can spoil surprises)!

Honestly, I really wish there was an upgrade path for the familiar itself, as chainlocks are very committed to their pets, but nothing about them will change throughout the game. I'd be much more likely to take a different familiar than Imp if it was going to get stronger later. As-is I feel I should think about level 14 and Imp is the only option that still seems compelling at that point. 

Thanks for the feedback!

Dnd is pretty vast and simultaneously very open to homebrew. That being said, reskinning the Imp would be the most simplest thing. So simple, you don't even need to bother 'creating' it, it's literally the same everything as the imp (though to nitpick, if you went that far, it should speak 'deep speech' and not infernal/abyssal).

Second, talk to your DM and create your own familiar. I forgot if it's in any source book but I think XGtE mentions that different warlocks/wizards can have different familiars; an undead hand crawler for necromancers/undying. a flying sword for hexblade, gazer for GOO, etc. This at least gives us suggestions that it is an option to expand the selection and suggestions on what we can use. Considering the vastness of the official content, use that for inspiration on creating a new one. We know that a standard familiar is what? 1/4 or 1/8 and that a chain one is up to CR 1? Look through the monster compendium in general, even non-relevant. See what stats, traits, features and abilities are granted to those monsters. This will give you a sense of what stats/features/traits/abilities would even be balanced for that CR. Look at monsters above and below CR 1 to see how they've advanced/regressed and take that all into consideration.

For example, I have a Kraken (Lurker in the Deep) warlock in my group and I've offered that if he took familiar from Pact of the Tome rituals I would allow him to take an octopus (with a fly speed to allow it on land) or if he took Pact of the Chain, I've actually created a 'tiny kraken'. Looking at the kraken and octopus statblock and taking into consideration that Chain familiars are stronger than regular ones I've made something that I'm pretty satisfied with. Similarly to the octopus it can grapple (but it's weak enough that you really don't want to be using this often, keeping it right next to enemies), It also has a 1x per short rest 'inky escape' causing darkness that only the warlock and familiar can see through.

For GOOifiying an Imp

As Noxx89 mentioned, change language from infernal to deep speech and maybe restrict it so although it also understands common it only speaks Deep Speech (resist the urge to give it telepathy)

change its appearance to resemble a tiny floating mindflayer (or Cthulhu)

change the sting/bite attack to a Psychic Blast Attack, uses action to target one creature, range 30ft, otherwise effects as per Vicious Mockery Cantrip (DC11 for save, if your DM allows it maybe switch it up to DC equals 8+Imps Cha mod + 1/2 Warlocks Proficiency bonus and damage as if cast by a Character of Warlocks level so it scales a bit)

change the shape-changing forms to: Octopus (so you can put it on like a "balaclava" and see in magical darkness without having a weird POV) and lose rat and spider forms and change to Swarm of Insects (worms or flies only to give it that Star Spawn feel form Mordenkainens Tome of Foes)

replace Immunity to fire with Immunity to Psychic damage

Great ideas! The reason I kept it mostly the same was to be "safe" in case of a DM that is worried about balance issues (I don't have a set DM for this character, and want to be able to use it in one-shots without complications, and if I'm recommending it to others, making it "safe" in this way seems like a good investment). 

Deep speech definitely makes sense, I didn't know what to put there but that sounds perfect. Making it not able to speak common would be a real downgrade, but not that bad assuming you have Voice of the Chain Master and can thus speak through its mouth anyway. 

Changing the poison damage to psychic totally makes sense. I might keep it as a physical attack for the sake of balance, but could change the flavor text to reflect that it's the touch of the creeping tentacles that causes the psychic damage. In terms of balance I feel like a LOT more creatures have poison resistance than psychic, so if anything it's a downgrade and unlikely to draw complaints.

What I really wanted was to give it fear like the Quasit has, that's another good option that probably would be balanced if I just gave up the poison entirely, but is a meaningful balance change since the fear power is the main thing that could be considered as better on the Quasit, which in almost every other way is worse than the Imp (HP/stats).  

In terms of immunity to fire, I'd say maybe this should have immunity to cold instead of psychic. It's not that devils deliver fire damage that makes them immune, it's that they are from hell where it's burning (at least that makes sense to me!). This creature would be from the depths of space (or the sea) where it's more likely cold than hot (e.g. the same "place" as Hunger of Hagar opens up, where you get 2D6 cold damage that this thing should be immune to). 

The animal forms are a can of worms. I am tempted to write it as "your patron chooses three mundane animal forms that please it" and let the player choose them to match the flavor they are going for. Personally I want a cat and a spider, so I'd choose that. Any two/three animals I could pick would probably feel arbitrary to a lot of people. I guess I could leave it open, but give examples of creepy animals other than raven/rat/spider, just to make it feel fresher. 

I really like your Cthulhu image you shared. I had already used an image that I thought could make sense as a "tiny Cthulhu". I'm thinking of how I could edit an image like that to show context and make the creature look smaller. By default the mind makes Cthulhu very large because of course :P

Oooh, this Forgotten Realms wiki page about Imps has a lot of interesting information! This part in particular:

If needed however, imps could polymorph into a variety of beast forms and individuals often had either one or two of them. Such forms were physically identical to normal animals and still allowed the imp to use their venom, although filtered through the animal's bite. Common animal forms for imps included that of a spider, rat, boar, raven or goat, although within raven and goat form they lacked the ability to use their venom.

To me that makes more sense than the default description that implies rat/raven/goat are the locked-in-stone forms they could take. Instead allowing the player/DM to choose 1-2 forms makes a lot of sense, and the examples listed are mostly just "on brand" for satanism (I really like the idea of a goat form!)

That makes me feel better about extending the premise to a GOO familiar: Allow the player/dm to pick two mundane animal forms that are on brand for the patron and use their movement speed whatever it is. 

The part about the venom is interesting, as it isn't addressed at all in the 5e imp description and actually has a meaningful impact on how you'd use that power.

Invisibility. The imp magically turns invisible until it attacks 

I had just assumed this meant: You can attack while invisible, but it immediately makes you visible and switches you back to imp. But looking now it doesn't say that at all. It also doesn't say if you can attack while in forms other than Imp (or how, e.g. whether the imp appears to be biting the victim or what). EDIT: I'm totally wrong about this, the Imp description calls the sting attack "Sting (Bite in Beast Form):" so it was already taking this into account!

Personally I think you should have to "expose" its true form to benefit from the attack. From a social perspective, having to keep your Imp/Aberration hidden and secret is part of the challenge of having such a powerful pet. Even your party members might not respect you anymore if they knew it's true form, so you are tempted to never let it be seen and that pushes you not to use the attacks. This is separate from the actual "wild" Imp of course. Getting a horrible poison bite from a spider and not even realizing it's an Imp messing with you is pretty cool.

For the sake of the homebrew GOO familiar, I think keeping it simple (and reducing the power) by having it only able to cause damage in its true form would probably be best, though I guess I could leave the text ambiguous like the Imp's. 

The wiki also says this:

They could turn invisible, and detect the presence of either magic or good at will. They also had the ability to use the suggestion spell once per day.

That sounds pretty good, but I suspect there's a reason that's not listed in the Monster Manual :P

Just as a heads up that wiki pulls the info from the 3.5e monster manual where imps were CR2 so a bit harder than 5e counterparts.

That said...I did just go take a look at my 3.5e Manual of the Planes book and they have a template for creating a "Pseudo" natural creature from the far realm which is where most, if not all, the GOO come from originally.

Basically its a set of abilities that can be added to any corporeal creature.

Trying to convert it over to 5e would allow a normal imp to have some extra abilities such as:

Creature gains aberration type

Can cast True Strike x1/LR (as per cantrip)

Alternate Form: At will the creature can use an Action to take the form of a grotesque tentacled mass or other appropriately gruesome form decided by the DM,  One creature of the creatures choice within 20ft. of it must succeed on a DC 10 Wisdom saving throw or be Frightened for 1 minute. The target can repeat the saving throw at the end of each of its turns, with disadvantage if the creature is within line of sight, ending the effect on itself on a success.

And those didn't increase the CR of the creature so you might be able to get a few other effects

 Hope it's okay to reply to this, just wanted to share the final form of this after my edits:

https://www.dndbeyond.com/monsters/559394-great-old-one-familiar

The character finally reached level 3 (we had to skip a few weeks) so it's now live. My DM was a bit nervous about having the Imp powers in our game (invisibility/shapechanger together with voice of the chain master for infinite range) but didn't see any reason not to let me reskin it as a GOO-relevant species. 

The main changes I settled on:

• Immunities: swapped to focus on cold and psychic immunity instead of fire and poison (it's from deep space like Hunger of Hadar, also probably fire and poison are MORE useful so it's a nerf if anything)
• Language: It knows Deep Speech instead of Infernal (torn between Deep Speech and Abyssal as neither one makes much sense, really it should speak something else entirely)
• Shapechanger: Changed the description to be more general and allow the player and DM to decide the alternate forms for roleplaying purposes: The familiar can use its action to polymorph into 1-2 beast forms from the Summon Familiar spell or back into its true form. The forms are chosen by the patron that creates the familiar, and the familiar's statistics are the same in each form except for movement speed(s). 
• Darkvision: Renamed Devil's Sight to Deep Sight just for flavor purposes, works the same. 
• Attack: Changed the bite+poison attack to be cold+psychic and be called Touch of Madness i.e. it rubs you with it's tentacle which is freezing cold and might make you go crazy a bit (especially if you can see the horrible thing!). Stats are the same otherwise

Maybe it will be useful to someone! If not at least I'll have some fun with the concept :)

 This is really cool btw.  Do you have a pic that shows what your creature may look like?  That Cthulhu image is pretty nasty.  

Ha ha well all my ideas are pretty nasty, I think it goes with the theme! If you had a DM that was into it, I think the DM picking the exact nature of your little horror would be a nice touch, since they could then describe it as part of RP during the game (and RP it's horrible discussions with you too). 

Here's a few of the H.P. Lovecraft inspired artworks that I found while thinking of my own abomination (which is based on the monster from The Dunwich Horror, which is invisible but made mostly of eyeballs and tentacles):

Their name is "Ojos" (spanish for "eyes")