History of Revenant Empowerment

Revenant Empowerment is the buff in Kell's Fall gained by collecting ten stacks of Revenant Essence. Stacks come from the darkness motes that Revenant Essentia drop when defeated in the mirror dimension (the Revenant Plane). Once you have ten stacks they convert automatically into Empowerment.

While empowered you can damage enemies that are otherwise shielded in the material plane, including the Pit Enforcers, Kaniks (the Mad Bomber) and the Hangman that Fikrul summons in the throne room. The mechanic repeats in the throne room: you collect another ten stacks in the mirror dimension, then return to the material plane.

Sources differ on the details. Destinypedia describes the Essence stacks as timed (45 seconds), while another guide says the buff lasts indefinitely. Check the in-game buff timer if the timing matters.