From the publisher's website:
When the dreaded black rain falls, all those who revere deities or rely on them for power and protection cringe in fear. The black rain forms a wall between the mortal and the divine and thus, when it falls, clerics lose all ability to cast spells and work miracles. Only the foulest of villains would use this opportunity to strike against the near-helpless clerics of an otherwise powerful temple.
When the black rain falls, the temple of St. Cuthbert comes under siege and a magical barrier prevents anyone from helping the beleaguered clerics and temple defenders inside. Someone has to find a way to aid them, and quick!
Black Rain, April's free online mini-module by Call of Cthulhu d20 RPG designer Monte Cook invites your 16th-level characters to challenge an insidious plot for revenge against the temple.
Imperfections mar the beauty of creation. In a battle among the gods themselves untold aeons ago, a discharge of divine might changed the weather patterns of the world so that once every few decades, an oily black rain pours out of particularly dark clouds. Whether by design or by mere mischance, this rain signals a period during which inviolate power severs the link between the mortal and the divine worlds. Even the gods themselves can do nothing to remedy this. Clerics lose all spellcasting ability, wards are suppressed, and so on.
During the time of a black rain, all characters lose the ability to cast divine spells or use divine spell completion items. Clerics lose the ability to channel positive or negative energy, and divine-based classes such as the paladin or druid lose supernatural or spell-like abilities (in the paladin's case, for example, this includes detect evil, divine grace, lay on hands, divine health, aura of courage, smite evil, remove disease, and turn undead).
Further, divine spell effects currently active when the rain starts, including permanent divine spell effects such as glyphs of warding, hallow, or bull's strength cast on a character, are all suppressed for the duration of the rain.
Terrid Fomgarten was a cleric of St. Cuthbert until his fellow priests found him extorting money from local merchants to protect them from criminals and evildoers. He was kicked out of the church and even spent some time in prison for his misdeeds. For years, he stewed in humiliation and regret, spending his waking hours training in martial skills to pass the time.
Terrid was not an evil man -- at least not to start with. Being a former Cuthbertian, however, he was prone to thoughts of retribution. Eventually one day, an erinyes devil named Yallathancia approached him in disguise. Over a fine dinner and a bottle of wine, she gave him two important bits of information that led him down the road to evil. She told him where he could find two artifacts of evil power and she let slip the date of the next black rainfall. With this information, Terrid set upon his own plan for revenge against the church of St. Cuthbert.
In his quest to get the artifacts, the iron rods, he encountered a powerful hydra and a number of creatures called bloodtrolls. Once he gained the rods, he used their power to master the monsters. Then, he bided his time for a few months, waiting for the moment when his foes would be at their weakest -- during the coming black rain.
Yallathancia showed him how to construct special rugs with teleportation circles on them. These special circles connected to each other like two-way gates. He kept one in his home, and one near the ruins where he found the artifacts (where his monstrous retinue stayed, including even Yallathancia). He made one to take to the temple with him on the day of the coming rain.
Finally, the day he waited for came.