A couple of additional observations about this encounter. You will first see 5 priests of Droga surrounding a pool. You might be able to avoid them, but you want to kill them anyway, as the final fight is in the pool. On the right side is a path, follow that path as it will take you to the balcony where the High Priest is. Kill the warriors on the way. Even if you get on the aggro list of a lot of warriors on the way, they are rooted in place, so just kill them one at a time.
You will get a message that more than 6 minutes has elapsed since the start of the ritual. It doesn't seem to hurt, although I suspect the final boss would not spawn if you killed the high priest within 6 minutes. Anyway, the High priest itself is immune to spells unless at point blank range (belly caster).
After the high priest dies The Incarnate Droga spawns below. When it gets to low health, it starts spawning golems with little health. We normally kill the first, and then ignore the second to finish off the boss. After the boss dies a chest spawns underneath the balcony.