It's pretty reasonable you're annoyed. One of your fellow players secretly plotted to kill your character for revenge (and it worked), the DM - the one guy you pretty much have to be able to trust - was in on it, and your fellow players offered you no emotional support at a point when you clearly needed it and instead made things worse for you.
People have been annoyed over character deaths, but you also have the issue of a betrayal of trust.
Different groups (and different DMs) handle stuff like this in very different ways - some handle it well, some badly. You had a conflict within the party, and a conflict between two people, and some character death and secret plots on your hands. These are often things groups don't talk about beforehand, but should. As the DM, I wouldn't have allowed this secret plot and would've talked to you two outside the game to get this enmity settled at first signs.
(And honestly, if this was real out-of-game racism you were experiencing, I wouldn't have tolerated that either.)
So now the guy who betrayed you is the DM and you want revenge.
Don't do it.
A good plan is to decline the invitation, not play, leave, and find something else to do or another D&D group to play with. This is not advice for how to ruin Bob's day. This is advice for how to avoid having your own next two months ruined, and possibly several weeks or months after that as well, and instead have some degree of peace for yourself.
You're pissed off. But, proverbially:
Holding onto anger is like drinking poison and waiting for the other person to die.
— (Not sure who first said this)
So you're going to have several gaming sessions and two months in which you're busy being pissed off at Bob, stressed out working out how to get revenge, and suffering over the lot of it - and expecting him to suffer for it. Eventually. At some point. Maybe.
That's not working out in your benefit.
Here's how the next two months are going to pan out according to this plan.
- Bob's the DM. The universe of the game you're considering bends to his whims.
- He doesn't like you to begin with, resents you for something you did to him, and apparently is subtly vengeful. He might give you a hard time for the next two months. This is going to feed right back into you being even unhappier in general, and unhappy to be in that game. This may even increase the degree of revenge you want, something you might not even get to begin with.
- You're going to be preoccupied being annoyed at him and not really actually just enjoying a good game of D&D. (Not that this game is going to be a good game of D&D for you necessarily anyway.)
- Eventually, you might actually find a way to get back at him. But, since he's the absolute controller of the game's universe, he can just say: "Oh. Well, my guy's goddess smiles upon him and heals him. Then she teleports your character into the plane of fire. Alright, whose turn next?"
- Or you don't get anything out of it and you're just annoyed.
- Or a month in, before anything even really happens, he or your fellow players ask you to leave because you're not being very fun to play with. You probably won't be. You're here for revenge, not to enjoy a good game.
- And you're playing with this guy as your DM the whole time, again. That's worth repeating. Why would you want that?
This entire plan is toxic to you and you alone. It's going to be really unpleasant for you, and more likely than not won't get anything out of it - and if you do, it probably won't be very satisfying.
Don't do it.
But I really want revenge!
This is an issue between you and another person, not an issue D&D will help with. Don't try to solve it through D&D, and don't try to solve it by playing with this guy as a DM for a few months.
Deal with it out of the game somehow. Maybe talk to him and get stuff off your chest. Consider walking away and leaving it behind you rather than let this weigh on you. Different things work for different people. I don't know what will work for you, personally. But spend these two months of your life doing something else. Find peace, somehow.
A surprising option is forgiving him - and not for his benefit, but for yours. Forgiveness is just as often so you can stop holding onto the negative emotions you have - the ones which are affecting you much more than anyone else - and let them go and find some inner peace. You might not be prepared to do that, but I advise you try it.
D&D is normally not like this.
(But sometimes it can be.)
There are going to be groups more supportive of first time players. There are going to be groups where you can genuinely be friends with most people, get along with all of them, and reliably trust the DM and your fellow players. This wasn't one of them, clearly. I suggest you find a group that suits you if you want to keep playing D&D.
One thing that normally goes unrecognised is that several D&D players at the same table, playing the same session at the same time, are usually not even playing the same game. They have different expectations of what a good game constitutes, different ideas of what's OK and what's not, different understandings of the rules, and different opinions over how issues like loot and character death should be handled. Usually, they assume that everyone else shares a similar view, without realising that everyone probably thinks very differently - to each other, as well.
An analogy is having several people sitting around a table to play a game of cards, but they are playing different games: one person is playing Poker, another Hearts, and another Go Fish. That wouldn't work very well, but somehow that's how D&D gets played without anyone realising it.
This disconnect is the reason why someone authored the Same Page Tool - which, as its name suggests, exists to get people on the same page. Its author also wrote about The Roots of the Big Problems and A Way Out (from which I drew the card game analogy). All three of these links discuss this issue and the situations that arise from it, and how to deal with them.
This group was not on the same page as you, and were not what you were after. Another group might be.
Limb loss is not a part of the game, there are no rules for it, and a DM who houserules it in is responsible for deciding what that means and how it is dealt with. Regeneration and the ring thereof are the only references to the idea, and they heal something that cannot happen. It's pure legacy cruft. The third edition of the rules went much more for the style of “fantasy heroics,” and does not want or need to accurately model injury in combat. Combat is the focus of the game and part of the fantasy is that these realistic risks are not risks for our great heroes or dastardly villains.
Furthermore, such a significant change to the rules relative to the default, not to mention the tone and style of game, should be announced ahead of time, as part of the premise of the game. You should already know the answer to your question, because your DM should have explained his homebrew limb-loss system to you before you made characters.
This is doubly true if your DM is using the lack of rules in this area as a “gotcha” response to your playing an undead character, which it certainly seems like he is. Did he tell you he was using houserules that included loss of limbs? Did he warn you that this was a weakness of playing an undead character when you brought the idea up? Because he very much should have, as this risk is not one present in the default game.
If he did, you might be out of luck; I think it’s a pretty poor thing for the game overall, but if you were aware of the risk and accepted that, well, that was the risk you took. I would strongly consider retiring the character at this point, however. A cripple does not make a good adventurer. It does depend on your class and such just how crucial it is, but most classes are going to struggle to the point where continuing an adventuring career would be suicidal.
If there was not warning, I would ask him what solutions are available, ask him out of character to provide a way for my character regain the use of both arms so that we can get on with whatever we were doing before you got crippled. A side quest or something. If this can be resolved, then fine, it's a part of the story and can be fun. I’m all for cool stories.
But with no warning, no communication of the change of rules and tone, and no opportunity to undo the damage, there’s a major problem. Your DM has introduced a massive, permanent, and unannounced change to a player character. If he just expects you to have this massive (and undefined) drawback for the rest of the game, I would consider that a major abuse of DM authority to spring such a large change on a player's character. I would at that point strongly consider leaving the game. This would not be a game I think is worth my time. Not really because the loss of a limb is unplayable necessarily, plus retiring the character is always an option, but this is a sign that this is a DM that oversteps what I consider his bounds to be, and is likely to do it again. I’m not interested into playing in a DM’s power trip.
Best Answer
Explain to him that "NPC" is not a derrogatory term
It seems to me you're already trying to do exactly this. You are, of course, entirely correct in that "NPC" means "Non-player-character". However, it seems your DM simply thinks that an "NPC" is any 'side-character'. Tell him that this is not the case. NPC's can be very important to the story, and can indeed be part of the main party. Hell, the DM deserves to have fun, and if the DM wants to have their own player character to obsess over and have fun with, they should be granted one. But that doesn't make the character less of an "NPC".
The dice are not omnipotent
You say your character is a smooth talker. As such, I do agree that this character trait should come into play. I agree with you that, no matter how important to the story and/or integrated into the party and/or fleshed out a DMPC is, it stays an NPC, and you should be able to make diplomacy checks. However, with the increase in character-depth also comes a clarification of boundaries. Not all NPC's were created equal. A shopkeeper who was made with just a physical appearance and maybe a general personality archetype can be swayed one way or another more easily than someone with rigid, written down morals.
The character has no background, and thus no underlying reasons to inherently agree or disagree with what you say. (Although they are still realistic people with lives outside of their shop even if the DM had not yet thought of these lives, and thus would not be willing to put their life or job at risk for a PC they never met without a pretty darn good reason. This reason, again, can be given with a good enough Diplomacy check.) A DMPC with rich and deep backstory, however, is different. If the DMPC is an Elf with a deep-seeded hatred for Orcs, which is rooted firmly in his background, then perhaps no amount of Diplomacy rolls may be able to talk this character into teaming up with an Orc. (However, I would not describe a character THAT stuck in his hatred actually fleshed-out, unless his stubbornness also has an underlying reason.)
On the other hand, however, that same backstory might be used to more easily convince the Elf to join the party on an Orc-hunting adventure. Having a character that is more fleshed out doesn't inherently make diplomacy harder or easier. It just makes it harder to convince the character to do certain things, and easier to make them do other things. The best way to find out which is which is to interact with the character and get to know them as a person.
Conclusion
You should accept that, as characters get more fleshed-out, they do indeed become more like PC's, which means that diplomacy checks might get harder, and some rare occasions may be too stubborn to listen to a diplomacy of any level. That does not take away that even the most fleshed out DMPC's are still NPC's, and should indeed be affected by Diplomacy as normal, within the boundaries of their characterization.