Diabolism -  Devotees of the Arch-Devils of Hell

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Benefits
The usual contract is for a "Wish" to be granted in return for the contractee's soul. Sometimes worshippers are given a "familiar spirit" (a minor devil, usually in animal form) to aid - and perhaps watch - them.

Life After Death
Immediately after death, the soul of a 'worshipper' is whisked away by minor devil to the relevant plane of Hell, there to suffer in torment for all eternity. Having contracted for the soul, the owning arch-devil can do with it as they see fit - usually this is to torture it (yielding the maximum soul-energy, or power), but it could be to place it in a devilish form, or to snuff it out of existence entirely.

SPELLS AVAILABLE TO CLERICS OF HELL
Level 1:    Bless, Command, Cure Light Wounds, Light, Protection from Evil, Remove Fear, Resist Cold, Sanctuary, Turn Undead*       
Level 2:    Chant, Find Traps, Hold Person, Resist Fire, Silence 15ft radius, Slow Poison, Speak with Animals, Continual Light*
Level 3:    Cure Disease, Cure Serious Wounds, Dispel Magic, Glyph of Warding, Prayer, Remove Curse, Speak with the Dead, Sunburst*
Level 4:    Cure Critical Wounds, Divination, Exorcise, Neutralize Poison, Protection from Evil 10ft radius, Flame Strike*
Level 5:    Commune, Dispel Evil, Plane Shift, Quest, Raise Dead, True Seeing
Level 6:    Aerial Servant, Blade Barrier,  Find the Path, Heal, Speak with Monsters, Word of Recall
Level 7:    Astral Spell, Control Weather, Regenerate, Restoration, Resurrection, Wind Walk