From the introduction:
In the aftermath of the Battle for Farzeen, the PCs realize that the time has come to press their advantage. By following the retreating forces of the ghouls and cultists, they come upon a citadel in what was an open square just days ago. The citadel, the product of the cultists’ fevered offerings, is an unwelcome surprise to the PCs. The fortress-like structure is both heavily guarded and nigh impregnable. Beating a hasty retreat, the PCs return to the subterranean tunnels for an alternative route of ingress only to find that the necrophages that plagued their earlier efforts have given way to a new ally of the cultists: the deep ones. Three custodians of the deep remain behind to guard the subterranean entrance to the citadel, each more powerful than the last. After defeating the final antediluvian protector,
the PCs find a way into the citadel.