Boneworm (CR 12)

Gargantuan Undead (Fire)
Alignment: Always chaotic evil
Initiative: -4 (Dex); Senses: Listen +5 and Spot +5

AC: 17 (-4 size, -4 Dex, +15 natural), touch 2, flat-footed 17
Hit Dice: 18d12 (117 hp); DR: 15/magic
Fort +6, Ref +2, Will +11
Speed: 30 ft.
Space: 20 ft./15 ft.
Base Attack +9; Grapple +17
Attack: Slam +14 melee
Full Attack: Slam +14 melee
Damage: Slam 3d6+13
Special Attacks/Actions: Desiccation aura, disease, dreadful chanting
Abilities: Str 28, Dex 3, Con -, Int 5, Wis 10, Cha 17
Special Qualities: fire subtype, SR 20, undead traits
Feats: Power Attack
Skills: Climb +15, Listen +5, Spot +5, and Survival +3
Advancement: 19-36 HD (Gargantuan), 37-54 HD (Colossal)
Climate/Terrain: Warm desert and underground
Organization: Solitary
Treasure: None

Source: Web

The dreaded boneworm is an amalgamation of dead desert-dwelling monks and priests. The insane tangle of bodies is fueled by a hatred of the deity and religion that failed to protect them all from starvation, thirst, and general insanity.

A boneworm is a massive creature that rarely measures less than 50 feet in length. Its wormlike shape consists of a mass of thousands of charred bones bound together by a thin membrane of tendons and scorched, leathery flesh. Hundreds of half-formed faces -- faces of the bodies the boneworm formed from -- press out of the leathery flesh at random points on the creature. These faces constantly contort into expressions of madness or rage, and they endlessly cry out twisted hymns and warped prayers to mock those deities they once revered.

The original boneworm came into existence when a large monastery located in the depths of a desert suddenly succumbed to a terrible disease known as bonefire. Some believe this disease came from one of the lower planes or even the Elemental Plane of Fire. In any case, the disease spread quickly, causing its victims to burn to death from the inside out as their skeletons became infused with terrible heat. It wasn't until after the victims died that the true purpose of the disease made itself known: The bodies of those who died of bonefire drew together and began to grow numerous new bones. The bones twisted together in a terrible braid of bone and dead flesh until it finally formed the monstrous abomination known as a boneworm.

Boneworms have little interest in communicating with anyone, but they do understand the Common tongue.

Fire Subtype

A creature with the fire subtype has immunity to fire. It has vulnerability to cold, which means it takes half again as much (+50%) damage as normal from cold, regardless of whether a saving throw is allowed, or if the save is a success or failure.