This is a build I worked up for a undead eidolon. Let me know if you think it is a bit much or not.
Undead
Base From: Biped (claws, limbs [arms], limbs [legs])
1st undead eidolons count as both undead and outsiders for the purpose of effects such as the bane weapon special ability and the favored enemy class feature. They gain a +4 bonus on saving throws against death effects, disease, paralysis, poison, sleep, and stun.
4th Undead eidolons gain a +4 bonus on saving throws against mind-affecting effects and immunity to nonlethal damage, fatigue, and exhaustion.
8th Undead eidolons gain immunity to death effects, disease, and poison.
12th Undead eidolons gain Burrow at their base speed and life-sense
16th Undead eidolons lose the +4 bonus on saving throws against Mind-effecting effects and gain immunity to ability damage, ability drain, energy drain, and mind-effecting effects.
20th undead eidolons gain immunity to paralysis, sleep, stun, cold