"I know just the place to gain plenty of Darkness energy for the Skeleton Key: Gambit, or as I have sometimes referred to it to poke fun, Drifter's Darkness Revue. Using Stasis against our enemies there should give you the boost you need." —The Exo Stranger
You can further empower the Skeleton Key by using Stasis to defeat combatants in Gambit, since they are brimming with Darkness. The Exo Stranger believes it will provide quite the boost towards empowering the key.