pf2e.fyi
From Pathfinder Impossible Magic, which isn't enabled.

Surprising Anatomy

REACTION
Activity reaction

Trigger An enemy adjacent to your eidolon deals damage to your eidolon

Effect Your eidolon's form shifts horribly to retaliate against the attacker. Your eidolon makes a melee unarmed Strike against the triggering enemy.
Pathfinder Impossible MagicORC