Hi everyone! I'm a bit new to D&D and am joining a campaign. For this game, I want to play as a plasmoid who can take human form. My idea is to start as a Warlock with the Undead Patron (my favorite subclass) and take the Mask of Many Faces invocation to roleplay a plasmoid who chooses to remain in a specific human form because they dislike their slime body. Even during combat, they would maintain their disguised appearance. I’m also planning to choose the Pact of the Chain for my pact boon.

I have a few questions about this setup:

1. I know Disguise Self can be given away by physical inspection or contact, but if I’m in combat and not actively hiding the fact that I’m a plasmoid, would the illusion dispel if I’m hit, or would the spell stay active and just reveal that I’m a plasmoid?

2. Are there specific classes or subclasses that can enhance the usefulness of a Pact of the Chain familiar? I’ve heard that chain familiar can become less useful in combat at higher levels.

3. Are there any feats that could make a chain familiar more effective, especially in combat?

4. What multiclass options would fit the theme of a plasmoid/slime character and add interesting flavor or should I stay warlock?

5. Do you have any recommendations for Undead Patrons, and how they could be integrated into my character’s backstory?(don't have much of an idea for backstory so any recommendation is fine)

I assume you're using the 2014 rules, because the 2024 Disguise Self is a Concentration spell (Edit: Disguise Self is not a Concentration Spell in '24, my mistake,.). And if you're not starting with a free feat for Eldritch Adept, I'm guessing you're starting at Level 2 or higher, since Mask of Many Faces isn't available at level one in either ruleset.

Whether Combat would risk you being exposed as a Plasmoid is an open question. You're probably safe in Combat from someone becoming certain you're a Plasmiod. I imagine there'd be a sense of "these stabs feel a little strange" or "I'm not hitting any bones when I punch/claw  this guy"  or  "This doesn't taste like human flesh, and has the wrong mouthfeel" which might be enough for a skilled combatant to become suspicious, but said skilled combatant would also have to survive the encounter, and spend an Action to make the Intelligence check. That said, someone discovering the illusion doesn't break the spell, so even if someone does figure it out, nobody else finds out automatically.

I think the major issue you're going to run into is sleeping, though. With Disguise Self needing to be cast at least once an hour, unless you're willing to spend a valuable Invocation Slot on Aspect of the Moon,  that's likely to result in plenty of situations where someone who finds you sleeping will realize what you are. Finding a way to discreetly cast the spell every hour might also get tricky. Overall, it's probably something you should discuss with the DM, to either get some magic item that will make it last longer if you can only use the single form, or make a single-form variant on the Invocation. And possibly with the other players.

As far as things that enhance a '14 Familiar,  anything that gives a reliable source of  Temporary hit points would be helpful such as the Inspiring Leader feat. Probably are going to have to accept that a '14 Familiar in the thick of combat, attracting attention to itself by attacking, is probably going to cost you 70 minutes and 10GP after each combat. Probably best to pick an Invisible Familiar, have it make Help actions for characters who are making Attack Rolls,  and have it retreat to cover after each attempt, if possible.

Rather than attacking, at higher levels of play I think it is actually better to have your familiar use the Help to give someone in your party advantage on at attack (a rogue in your party will love you for this). If you have an Imp, a Quasit, or a Sprite for a familiar, they maintain invisibility while giving the help action, drastically reducing the chances that a monster will damage them.

I also really like the Voice of the Chain Master invocation with any of these familiars. This invocation lets you speak through your familiar with your own voice, but also lets you see through their eyes with unlimited range so long as you are on the same plane of existence. So now you have an invisible scout whose eyes you can see through at unlimited range. It is basically an unlimited range, no cost Arcane Eye, that you can also use as a Sending spell in a pinch. Pretty versatile for one invocation cost.

Thanks for your input. Yes, we will be using the 2014 rules, but I have never seen in the 2024 rules that disguise self-require concentration. Can you link where you found that info? I looked it up, and I don't see where it requires concentration.

Yeah, I guess that's a common feedback. I was hoping there was a way to use it as a combat summon, but using it for the Help action isn't too bad, especially with my luck rolls. Thanks for your input!

Quote from TLDjr2005 >>

Thanks for your input. Yes, we will be using the 2014 rules, but I have never seen in the 2024 rules that disguise self-require concentration. Can you link where you found that info? I looked it up, and I don't see where it requires concentration.

Huh. I must have been looking at the wrong spell. I could have sworn it did.  Apologies.