You were deep within Castle Volkihar. You'd been to the abandoned house in Markarth, so you knew the altar of Molag Bal when you saw it. Serana had already left the room, her father's ashes on the floor. The mace in your pocket seems to scream at you. Your head is in pain until eventually you fall to your knees. Finally, a voice booms in your head.

Molag Bal: "So, you've found my people in Skyrim, champion. Very good. Those vampires are too confident in themselves. Let the mace show them submission. Then go back to your hunters and show them what it means to kill the creations of Molag Bal." His voice is eerily smooth as he commands you to beat down both sides. The altar stares down at you, commanding your compliance. Finally, he speaks his reward. "Do my bidding and you'll have a place above most in Coldharbour." With that, the ringing falls silent, falling off as he laughs sadistically.