I think that forcing one stat to be constant is a bit harsh.
When we dramatically change the core mechanics for a race the results might be too different from races of other players. This means that either the difference must be small or they must require other resources that are comparably available within the game.
Endurance/Resistance does fit Undead very well, the thought pushing it onto hp might be too concrete already.

Now i will flood this thread with my other ideas:

- I just take what I need: Every active or earned point of source increases hp.
- The End is...still not the End: Survives the first fatal blow every fight.
- No one likes you: +20% Resistance against everything, -20% hp
- Where is my kidney?: Immunity to critical strikes
- Blood is thick: Most Undead prefer not to attack you.
- Stand-Up-Veteran: Immunity to sleep and exhaustion



EDIT:@stabbey I like this concept; it makes Undead more gear-dependent and allows for high-hp-builds. And you can easily make middleground between this and standard.