Not totally broken
The easiest way to determine if this is any good is usually to compare to other races - so lets go item by item
Ability Score Increase. Your Constitution score increases by 2, and your Dexterity score increases by 1.
+2/+1 is in line with many existing races:
- Elf: +2 DEX / +1 [INT/WIS/CHR]
- Halfling: +2 DEX / +1 [CHR/CON]
- Dragonborn: +2 STR / +1 CHR
This seems to follow the baseline stats of many D&D races.
Size. Skeletons match the height, but are half the weight of humans. Your size is Medium.
Speed. Your base walking speed is 30 feet.
These are fairly standard, although it is worth noting that some races such as Dwarf/Halfling which have relatively strong features are limited down to 25ft base movement. If you decide that the race is perhaps a little strong this is somewhere I would consider adjusting it slightly.
Darkvision. Thanks to your undead nature, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
This is quite standard, but it's also worth noting that not all races have access to this. Aarakocra, Human, and Dragonborn do not (although many do). If we determine this race to be quite strong I would consider removing this small feature, to encourage the character to take the light cantrip or carry a torch.
Body Construction. Because of your relentless undead fortitude your body is capable of detatching and reattaching limbs. After a long rest you can choose one of the following abilities:
- Detach Hand.
- Detach Head.
- Detach Arm.
- Attach Extra Bones.
These seem thematically cool, and mostly not too strong. the Detach spells seem mostly like interesting cantrip-level spells as far as I can tell, and add flavour to the character.
Attach Extra Bones Seems more powerful however, and could be quite the boon to certain character types. Gaining AC is generally a powerful effect, many characters carry a shield for +2AC and this means they lose the use of one hand for combat. Dual wielding or using a Duelist style can add +1AC for certain character types which is decent in many cases. For higher level characters this can mean gaining more than the benefit of a shield for free. For a monk character type specifically (who cannot use a shield) this power could be very broken.
I would likely advise removing the AC component of this spell, or making one (or more) of the following changes as a bigger down side related to the bonus:
- Half movement speed
- Disadvantage on stealth checks
- Disadvantage on dexterity saves
False Appearance. While you remain motionless, you are indistinguishable from a pile of bones.
Pretty cool, if your PC gets a chance to use this it will be pretty fun.
Necrotic Constitution. You have resistance to necrotic damage, and you have immunity to poison damage, the exhaustion condition and the poisoned condition. You also have vulnerability to bludgeoning damage.
Having a resistance is usually quite strong but not unheard of. Dragonborn, Dwarves, and Tieflings all have resistances to one element.
Immunity is very strong mechanically, although poison is not the most common damage type from enemies.
Vulnerability is however quite a large risk factor and may help balance out the strengths here. Thematically you may want to consider also including the radiant damage type as another nerf for this character. I might suggest something like this:
You also have vulnerability to radiant damage, and whenever you receive bludgeoning damage you receive an additional 1d8 bludgeoning damage.
Undead Nature. You skirt the line between life and death, you count as both humanoid and undead when it comes to spell effects and abilities. You can’t and don't need to eat, drink, or breathe.
Also quite strong. In a survival situation not needing to eat could be big, however this depends on your DM and how they handle survival.
The main problem here is not needing to breathe - this means that your PC can go underwater indefinitely, and may be immune to the effects of certain gas effects or spells. This could also be totally useless depending on what kinds of challenges the party faces, so it's situational - but in those certain situations it is very strong.
Languages. You can speak, read and write common and one other language of your choice.
Seems quite standard for many races.
Summary
Overall everything here is either in-line with other races, or slightly stronger, or situationally stronger. As the dungeon master, it's up to you to decide what you are happy with. My suggestion would be to make a few nerfs, but I don't think the character is totally broken and I think you could run with it.
My Suggestion:
Definitely do something about Attach Extra Bones, I think the additional AC is over the line and is probably the only thing that really is too much.
Probably choose at least two (possibly more) nerfs to take some of the advantage to disadvantage:
- Lower the base movement to 25ft
- Remove nightvision
- Give disadvantage on stealth checks (skeleton feet probably make a similar noise to walking in high heels)
- Remove some immunity/resistance, or increase vulnerability/extra damage
- Require the PC to breathe
- Lower the ability score increase by 1
Madam Eva's motives are complicated
First, regarding your notion that Madam Eva is in league in Strahd as she is a Vistani in the Tser Pool, note the description of Madam Eva from the appendix D:
The fortune-teller Madam Eva lives among the Vistani but isn't truly one of them.
Further, the true focus of Madam Eva seems to be to:
Find a worthy replacement for Strahd. From the appendix description: "She would rather help Strahd find someone else to succeed him, although she has grave doubts about her ability to locate such an individual."
Because of this, her actions and motives can seem to be a bit contradictory, note this description also in the same appendix (emphasis mine):
In the guise of Madam Eva, she uses this ability to help Strahd. She can send her Vistani out in their wagons to visit other worlds and bring adventurers to Strahd's domain, in hopes that they will find a way to destroy the vampire or set Strahd free.
So she does want to help him, but the way she wants to do so does entail helping the adventurers and you can't exactly say that she's in league with him in the same way the Vistani are.
Best Answer
It is unclear where the extra damage comes from
Dealing additional damage in this way is perfectly normal for creatures of this type as seen in their basic statblock:
It is fairly clear who the NPC's adversary is:
However, only this NPC applies the damage in this specific way, as the other NPCs of this type do not get this benefit:
That being said, the damage may still come up as shown below:
Not every fight is to the death
There may be instances where the NPC would have to fight Strahd with no intention of killing him. In these cases, his weapon is all that more effective. Some of these instances could include (but I'm sure there are others):
Nothing explicitly forbids a party from compelling him
A highly proficient effort could be made to convince the NPC to abandon his original mindset and help the party against Strahd.
A DM could rule that this is outside the realm of possibility for the party, but equally valid is simply assigning a particularly high DC to any relevant ability checks. The Basic Rules recommends a DC of 30 for "Nearly Impossible" efforts, and this is where I personally would assign the DC.