World's Largest Dungeon
The chapel is a large room with two rows of pillars. At one end of the room was a portal to a demi-plane in which hundreds of fiendish creatures (Darkmantles, rats, and the like) were sealed. At the bidding of his familiar, Aschyyx, Longtail released these creatures, who now plague region A.
The demiplane also contained more dangerous creatures, such as a fiendish owlbear and even a bebilith. The former was killed by Thokk and Misty, while the latter was unable to escape as Cygoponi and Fuujin sealed the portal just in time.
Whether or not the portal can be re-opened again, and how this would be accomplished, is currently unknown.