The Ancient Dark Beast is the second boss that you come across if you move through the Sanctum of the Burning Souls in the preferred order. Follow the path and you will see a two direction split, one leading up an altar, the other leading away from the altar. You must travel up the altar to reach the Dark Beast, but first you will notice some Nemedians and some Black Ring Members fighting each other. If you are quick enough you can sneak along the far side to avoid conflict with either faction, but if not you will have to fight through the middle. Upon reaching the altar, and if one of your groupmates picked up the item that dropped off of the Vengeful Drowned One, you can use it upon the altar to summon him, for a quick and easy tank and spank fight.