Human male. He’s dressed in dirty rags and a shredded brown overcoat, and looks like a decrepit homeless lunatic. He’s clearly infected with something. His skin color is wrong, his eyes are sunken and hollow, and his skin is bubbled and chaffed raw. (In the picture, ignore the giant beehive hump he has).
He speaks in rasping mutters, and seems to be the leader of the company of villains you met at the ziggurat. Seems to have a thing for insects/swarms. With the completion of his latest terrible ritual, his consciousness is now divided among hundreds, if not thousands, of beings.
Swarm has taken managed to use the power of the crystal bugs and the black onyx to obtain the power to become a swarm. He can now possess any creature.