"Clovis left behind a Skeleton Key to the secrets of Europa, but never told me how to use it. In my dark future, I figured out it needed to be empowered with different types of energy. I used it to unlock Clovis's journal then. But in this timeline, it remains locked." —The Exo Stranger

The Skeleton Key requires energy to empower it. The Exo Stranger believes that defeating Fallen and using Stasis abilities should help you to do so.