In a battle among the gods themselves untold eons 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.
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 their abilities 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.