Below are my thoughts for clerics serving the God Worm, based on Tony Dowler's Purple Worm Graveyard. In the adventure, it's implied that there's a tradition of human cultists worshipping a powerful Worm God, and that it usually goes badly for them.
I liked the implication that there might be good reasons for humans to worship a giant worm, and that it might still be a terribly bad idea for them to do so. Worms are the eaters of the dead, so the God Worm bestows all the usual clerical powers, albeit with some variations in their appearance. Servants of the God Worm are also invariably destroyed by their connection to such a powerful inhumane entity. I've added the option for clerics to choose to become tainted, instead of accepting the usual consequences of deity disapproval, and I've added a very tempting offer, for clerics to burn away their minds instead of their luck, for a larger one-time benefit, but at a cost that's irrecoverable.
CLERICS AND THE GOD WORM
Alignment: The God Worm is neutral in (or perhaps prior to and separate from) the great Cosmic Struggle between Order and Chaos, but its worshippers may be of any alignment.
Holy Symbol: The cleric's holy symbol might be a wood staff carved to look like a worm, an ourouboros (a symbol of a worm making a circle and biting its own tail) or an auryn (a symbol of two worms making a knot and biting each other's tails), or a live worm that she keeps as a mascot.
Lay on Hands: Whenever the cleric calls on the God Worm's power to heal the injured and sick, her healing touch manifests as a swarm of maggots, leeches, and other medicinal vermin. These fall from the cuffs of her sleeves, sweat from the pores of her hands, and burrow up through the skin of the patient. They eat any infected or necrotized flesh, leaving a clean sterile wound, and disperse at the end of the healing.
Turn Unholy: Whenever the cleric calls on the God Worm's power to turn away the unholy, the primary manifestation is of worms and maggots rising halfway up from the soil to repulse the enemies of her faith. If the turning attempt includes a holy smite, this manifests as a stream or cone of devouring worms flowing out of the cleric's hand and drilling into her foes.
Deity Disapproval: Whenever the cleric rolls within her current disapproval range, she may choose to receive patron taint from the God Worm instead of accepting the result of the disapproval roll. She may choose to receive patron taint after seeing the disapproval result, but after this choice is made, it cannot be reversed based on the patron taint roll.
Burning luck: Clerics who serve the God Worm may choose to permanently burn their Personality or Intelligence ability scores instead of burning Luck to modify the effects of laying on hands, turning unholy, or spellcasting. The God Worm rewards this alignment of the cleric's mind with the annelid consciousness by granting a +CL bonus to the modified roll in exchange for this sacrifice. Clerics who serve the God Worm can never increase their mental ability scores; any effect that would ordinarily increase a cleric's Personality or Intelligence scores increases her Luck score instead.
Blessing, Holy sanctuary, Protection from evil, Divine symbol, Spiritual weapon, Sanctify: Spells that confer a blessing or protection manifest as an anointing of worms crawling over the affected person, object, or place.
Detect evil, Detect magic, Second sight, True name: When the cleric casts spells that allow her to commune with the mind of the God Worm to gain knowledge, she must accept patron taint if she rolls in her disapproval range, as described above.
Food of the gods: The God Worm sends edible meal worms for the cleric to eat.
Cure paralysis, Neutralize poison or disease, Restore vitality, Remove curse: Healing magic is affected as the laying on of hands, described above.
Snake charm: The affected snakes shed their skins to reveal snake-sized earthworms beneath. This change persists even after the spell's duration ends.
Stinging stone, Bolt from the blue, Affliction of the gods, Vermin blight, Whirling doom: Spells that afflict the cleric's enemies manifest as an attack by worms instead of other creatures or objects.
Wood wyrding, Cause earthquake, Desecrate: The God Worm sends burrowing worms to drill though the affected object or piece of land, and their movement creates the effect of the spell.
Animate dead, Speak with the dead: The God Worm does not allow its clerics to animate or speak with the dead. If the cleric's random determination of spells would result in her learning either of these spells, she re-rolls to learn a different spell instead. Attempting to cast these or similar spells results in automatic failure, deity disapproval, and patron taint.
Other magic: Clerics who serve the God Worm manifest their spells differently than other clerics. The player and the judge are encouraged to devise additional alternate manifestations for these and other clerical spells.