To receive the final relic from the Lord of Hollows and get the “A Parting Gift” trophy/achievement, you must watch for the whirlwind while you’re on your way through The Scar. Use the whirlwind to reach a higher area that will lead you to Abraxis. You must kill Abraxis (optional boss battle) and then go to the Lord of Hollows’ Chamber. Spare the Lord of Hollows to receive the relic when he disappears.
Sparing the Lord of Hollows also unlocks a different ending scene, in addition to changing some of the dialogue in the ending. This is considered to be the best/true ending.