Comments from the Finger: I originally scrapped my draft of this in favor of the Dream Domain, which I felt was a truer representation of Lovecraftion fiction, but Rex Gunn helped me out on resurrecting this one!
Your power as a cleric comes from the divinity of the madgod. Every paranoid thought and nervous breakdown in the world empowers the madgod, and you as his emissary. Of course, little do people know that the madgod is just a cobbler who lives down the road and smiles kindly at passersby. Well, actually he's a little magpie that flies around at daybreak. Or perhaps he's a wedge of cheese that talks to the clouds?
Madness Domain Spells
1st dissonant whispers, Tasha's hideous laughter
3rd crown of madness, phantasmal force
5th clairvoyance, hypnotic pattern
7th confusion, hallucinatory terrain
9th dream, modify memory
When you choose this domain at 1st level, you gain proficiency with martial weapons.
At 1st level, whenever you make an ability check, you can choose to use your unique insight to make connections which may or may not exist. If the d20 roll for the ability check was even, you add your proficiency bonus to the check, even if you have already added it. If the roll was odd, you subtract your proficiency bonus from the check.
Channel Divinity: Give Madness
Starting at 2nd level, you can use your action to force a creature within your reach to make a Charisma saving throw or be deranged for 1 minute. Whenever a deranged creature makes an attack roll and rolls an odd number on the d20, the attack misses.
By 6th level, your damaged mind is difficult to influence. You have advantage on saving throws against being charmed or frightened, and on saving throws against any effect that would sense your emotions or read your thoughts.
At 8th level, you gain the ability to infuse your weapon strikes with divine energy. Once on each of your turns when you hit a creature with a weapon attack, you can cause the attack to deal an extra 1d8 psychic damage to the target. When you reach 14th level, the extra damage increases to 2d8.
Starting at 17th level, whenever you use your Channel Divinity to give madness, you may choose to give the target a randomly rolled short term madness effect (found in the in the Dungeon Master's Guide, page 259). After using this ability, you must finish a long rest before using it again.
Changelog: 8/8/16: Give Madness: Simplified to use the d20 roll of the attack roll. Reworded slightly.
Lunatic Insight: Simplified to use d20 of ability check for the random effect