Defeat the Shade inside you within three and a half minutes.
The Once and Final King is obtained by defeating the Shadowlord within three and a half minutes. Hit him with Dark Hand if in close proximity or Dark Lance from afar. Once his health is half depleted, he starts flying, so use Dark Lance until his next phase. For his final phase, the Shadowlord will be at the end of the room and shoot a barrage of magic orbs at you. Evade them and attack him with melee until he teleports you to the other side of the room. Rinse-and-repeat until he is defeated.