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

Phase Out

FEAT 6
Activity 3 actions
Prerequisites your eidolon is a phantom

Your eidolon phases out of reality slightly, gaining resistance to all damage (except force, spirit, and vitality) equal to half your level. They're not incorporeal, but can't use Strikes or actions that require a fully physical form, such as Grapple, Reposition, Shove, or Trip.

Your eidolon can Dismiss this effect to return to their normal form.

Pathfinder Impossible MagicORC