An eidolon appears as an undead creature, and mimics some of an undead creature's abilities and weaknesses. Negative energy heals the eidolon, and positive energy (including a cleric's channel energy ability) harms it. Spells and effects that target undead or have specific effects against undead (such as Command Undead, hold undead, and searing light) affect the eidolon as if it were undead. The eidolon gains a +2 bonus on saves against disease, exhaustion, fatigue, paralysis, poison, sleep effects, and stunning.
At 7th level, this bonus on saves can be increased to +4 by spending 2 additional evolution points. At 12th level, this protection can be increased to immunity against these attacks by spending 2 additional evolution points (the summoner must pay for the 7th-level upgrade before paying for this 12th-level upgrade).
Although the eidolon appears undead, it is still an outsider.