The story takes place in a village in Ravenloft. The party has been invited to accompany the young couple Burgomaster Alexandru and Mihaela Nichitovich [ni-KEE-toh-vich] of the hamlet of Nichitovia [ni-kee-TOH-vee-uh] to Rosenwald, a deeply religious community. Alexandru has asked the priests of Rosenwald to conduct a traditional ritual to gain him the favor of the Morninglord for his tenure as burgomaster. This is a ritual all burgomasters traditionally receive, including his late grandfather, who is entombed at Rosenwald. Father Petre and Sister Elena of the church of the Morninglord loosely run Rosenwald. They offered to throw Alexandru a banquet at the House of the Morninglord before the ritual, set to take place during the next full moon. However, all of the clergy have secretly become afflicted with the aswang curse. Father Petre is a halimaw and Sister Elena a kubot, both of whom gained their powers through dark pacts with demon lords of the Abyss. The other priests and acolytes are busos, survivors of encounters with roaming tigbanuas, who found sanctuary with Petre and Elena. The pair refer to these busos as their flock, and plan to feast on the guests after the banquet.

The Dysmal Swamp is a coastal wetland that stretches over 100 square miles. Devoid of any urbanization, the ecosystem enjoyed its dank sanctuary. A community of bullywugs called the swamp home for generations, hunting and maintaining the balance of the land. The land also appealed to the Belange Company, a wicked fey-run enterprise making cheap magic items, who needed a secluded location far from both thieves and oversight. When the Belange Company established their magical workshop in the swamp, the bullywugs attempted to drive them off. However, the Belange Company’s supply of magical items provided enough firepower to protect their endeavor, and many bullywugs fell in the initial conflicts. The Belange Company began crafting scores of magical items to be sold across the region, using a cheap alchemical process that produced a nasty liquid byproduct glowing with chaotic magical energies. Rather than devise a new process, the Director decided to discharge the waste into the swamp. As production increased, so did the pollution, and the sludge began to warp the wetland. The region’s typically reclusive bungisngis population became aggressive. It is only a matter of time before these mutated swamp giants lay waste to bullywugs.

This adventure is written with the intent that it takes place in the subterranean world called the Underdark in the world of Forgotten Realms, although it may be placed in other settings. Within the Underdark, Cyrog's lair can be placed anywhere though it is stated to be thousands of miles east of Gravenhollow. If you are running Out of the Abyss, you may want to have your players see the vision Cyrog Lives! Hail Orcus! (OOTA, 158). Additionally, it is recommended that this adventure be used for a party of 4 to 6 players, level 9 to 12. While merely hinted at through visions in Out of the Abyss, Orcus is roaming the Underdark unchallenged with an army of mind flayers at his disposal. Cyrog, the elder brain of a mind flayer colony in the Underdark has died. The demon lord Orcus has reanimated and taken control of it. The hive of mind flayers is now bent to Orcus's will through Cyrog as he strives to make all life in the multiverse into a vast undead horde under his control. The adventurers are contacted by a patron who gives them a special item (such as brain worms) to suppress their brain waves and make it possible to infiltrate the Cyrog's Lair without immediate detection. The adventurers must defeat or otherwise subdue mind flayers and a few guards, override the elder brain's mechanical defense system, and slay Cyrog.

The party seeks out Commodore Krux at the Happy Beholder. After speaking with the patrons, the party learns that Krux has disappeared and gets a lead to check out his ship, the Second Wind. At the ship, the party learns from Fel Ardra and Flinch that Krux is being held prisoner by the Amoebros in a cave on the underside of the Rock of Bral. The party infiltrates the Amoebros’ lair, gets past the guards, and learns of the animal experimentation performed by the gang. In a final climactic scene, the party rescues Krux from Ripples, the wicked boss of the plasmoid gang.