Nefreet |
First Worlder was printed in Inner Sea Magic, way before Pathfinder Unchained was released.
The 2015 Society Unchained Blog wrote:
The unchained summoner qualifies for all existing summoner archetypes, save those that modify the eidolon's type or base form.
First Worlder modifies the Eidolon's creature type from Outsider to Fey, so it is incompatible with an Unchained Summoner.