There is also a cool prestige class in it called the Doomdreamer.Return to the Temple of Elemental Evil page 161 wrote:THE INSANITY SCORE
Regardless of whether they select the Madness domain, all clerics of Tharizdun and the Elder Elemental Eye share one common aspect: They are insane. To reach beyond the veil and draw upon the power of Tharizdun is to touch madness itself, and no one can do so and come back unchanged.
As a special rule, every cleric of Tharizdun or the Elder Elemental Eye gains an Insanity score equal to half his cleric level (count any doomdreamer levels as cleric levels for the purpose of calculating this score). For spellcasting (determining bonus spells and DCs), add this score to the cleric’s Wisdom score and use the result in place of Wisdom alone. For all other purposes, such as skill checks and saving throws, subtract this score from the cleric’s Wisdom score and use the result in place of Wisdom alone. This means that the spells of the servants of the Dark God are very difficult to resist, but those servants are in general unaware of their surroundings and act imprudently—often erratically.
This mechanic is for lovecraftian or end-of-world type of clerics, not for the Lawgiver or Eternal Order clerics. Its a great addition to Ravenloft.