Phase Out Feat 6
Source Pathfinder Secrets of Magic
Prerequisites your eidolon is a phantom
Your eidolon phases out of reality slightly, gaining resistance to all damage (except force and negative) equal to half your level. It's not incorporeal, but it can't use Strikes or actions that require a fully physical form, such as Grapple, Shove, or Trip.
Your eidolon can return to its normal form with a single action, which has the concentrate trait.
Traits
Anything that doesn't list another rarity trait (uncommon, rare, or unique) automatically has the common trait. This rarity indicates that an ability, item, or spell is available to all players who meet the prerequisites for it. A creature of this rarity is generally known and can be summoned with the appropriate summon spell.
ConcentrateAn action with this trait requires a degree of mental concentration and discipline.
SummonerThis indicates abilities from the Summoner class.
TransmutationEffects and magic items with this trait are associated with the transmutation school of magic, typically changing something's form.