When you grab the Dark Phylactery it starts this quest. Go back the way you came and up into the cave to the west. There is an Altar in the middle of the room. the mobs around the walls will not bother you. Run forward and click the altar to summon Angdralthus the Lich, then kill him. After, Alsbeth the Discordant and speaks to you.
"Very good, Ascendant! You have slain Angdralthus, but too late. We already have all the sourcestone we need for the ritual. Soon, Regulos will beset free upon Telara!"