Bring the Skeleton Key to the Exo Stranger in the Bunker E15 Lost Sector in Eventide Ruins.
The Skeleton Key is fully empowered. It's time to rendezvous with the Exo Stranger.
"The Skeleton Key's ready. Come find me at my family's secret bunker and use it to unlock the journal. It's time to expose that which Clovis desired to stay hidden." —The Exo Stranger