SRD:Eidolon

Upon casting eidolon, the character creates a duplicate version of him or her self as the character was when he or she was a 21st-level character, and the character gains one transform** negative level while the duplicate persists. For each additional transform** negative level the character bestow upon him or her self at the time of casting, the eidolon has one additional character level. No matter how many transform** negative levels the character bestows on him or her self, the eidolon can never have more character levels than the character has (taking the transform** negative levels into account). Treat the duplicate as the character with a number of transform** negative levels conferred that would lower him or her to the character level of the eidolon. The eidolon is considered fresh and rested when created. It may cast any spell the character has access to, including an epic spell. Use the eidolon’s Spellcraft modifier as the basis for the number of epic spells it can cast in a day, and its effective character level as a basis for its skills, feats, and other abilities. The eidolon is effectively lower level than the character and probably can’t cast all the spells he or she knows. A powerful enough eidolon might conceivably cast the eidolon spell itself. The eidolon appears in whatever mundane clothing the character desires when initially conjured, but it has no other possessions. It shares part of the character’s soul, so it is the character for all intents and purposes. The character and his or her eidolon communicate with each other normally. Usually, the eidolon does not begrudge its brief existence, because it is still part of the character. If the eidolon is killed prior to the expiration of the spell’s duration, the character immediately regains the lost levels. Normally, the eidolon does not last long enough to threaten the character with permanent level drain.

XP Cost: 2,500 XP.