Rend eidolon attack.

[Alex_UNLIMITED]

If my eidolon have energy attack evolution and a vicious amulet of mighty fists, when he rend, what happens?

[Mabven the OP healer]

Rend is a flat bonus to damage when you hit with 2 or more claw attacks. It is not a separate attack, thus you do not add any additional energy damage, or any other damage bonuses for that matter.

Think of it as bonus damage added to your last claw attack, and thus, is also not subject to any damage reduction which your last attack overcame at all, even by just one hit point.