Defeat the House Salvation Fallen platoon near the Crux of Darkness in Creation.
"One final burst of energy, and we'll have what we need to face the Clovis AI together. There's a Fallen Captain that's been making its way towards him. Taking it down should be all you need." —The Exo Stranger
The Skeleton Key is almost ready. The Stranger believes one more boost of energy from combatants near a Crux should do it.