Sacred Haven of Debauchery

What tricky creatures did I encounter in Battlegrounds? In the "garden-labyrinth" room, there are ghouls running around, along with some invisible sirens. They are completely invisible, approach you at close range, and deal significant damage, capable of killing you in two or three hits, and then they become invisible again. Therefore, I summoned skeletons and sent them ahead, while I followed behind. The invisible creature approaches the skeleton, starts attacking it, and we immediately all shoot at it. Then, I got tired of moving with a crowd and took only an archer and skeletons. That's how I finished them all off. Just as the last skeleton was about to die, the archer finished off the last invisible creature with an arrow as it approached him.