Necroclasm (CR 25)

Colossal Undead
Alignment: Always chaotic evil
Initiative: +8 (+4 Dex, +4 Improved Initiative); Senses: darkvision 60 ft., Listen +30, and Spot +30

AC: 29 (-8 size, +4 Dex, +20 natural, +3 deflection), touch 9, flat-footed 25
Hit Dice: 50d12 (325 hp); DR: 15/good
Fort +16, Ref +22, Will +27
Speed: 30 ft., fly 80 ft. (perfect)
Space: 30 ft./30 ft.
Base Attack +25; Grapple +58
Attack: Slam +34 melee (4d6+17/19-20)
Full Attack: 4 slams +34 melee (4d6+17/19-20)
Damage: -
Special Attacks/Actions: Fiery aura, incineration, thunderous roar
Abilities: Str 44, Dex 18, Con -, Int 6, Wis 6, Cha 16
Special Qualities: immunity to electricity and fire, SR 26, undead traits, vulnerability to cold
Feats: Alertness; Ability Focus (thundrous roar); Awesome Blow; Cleave; Combat Reflexes; Dodge; Flyby Attack; Great Cleave; Improved Bull Rush; Improved Critical (slam); Improved Initiative; Improved Overrun; Improved Sunder; Iron Will; Lightning Reflexes; Mobility; Power Attack; Whirlwind Attack[B]
Skills: Balance +8, Intimidate +23, Jump +21, Listen +30, Spot +30, and Tumble +30
Advancement: 51+HD (Colossal)
Climate/Terrain: Any volcano
Organization: Solitary
Treasure: Double standard (no flammable objects)

Source: Web

What at first appeared to be a fast-moving cloud of volcanic ash resolves into something far more terrible. Thousands of burning bodies and blackened skeletons tumble and roil in the cloud. Periodically, huge tentacles formed of ash and bodies emerge from the cloud to swipe away buildings and great swaths of trees. The unliving bodies emit a constant thunderous scream, as if a volcano had learned to wail in agony from the fire in its core.

The necroclasm is one of the largest forms of undead; fortunately, they are limited in range to the volcano that spawned them. A necroclasm forms when a large group of evil creatures are destroyed by a volcanic eruption. Typically, a city of evil-doers founded on the slopes of what was assumed to be a dormant volcano form the building blocks of a new necroclasm. The creature itself is, in some ways, similar to a swarm of burning undead bodies, although the creature displays few of the actual qualities of the swarm subtype.

Necroclasms possess some intelligence, but they have little interest in turning their evil outward into surrounding regions. They prefer to haunt a forty to sixty mile radius about the base of their parent volcano, and they constantly scour the landscape for living creatures to eradicate and absorb into their mass. A necroclasm must retreat to the cauldron of its parent volcano during the night; unlike most undead, these creatures are active only during the day. They retain their abilities and mobility at all times within the volcano itself, but they cannot physically move beyond the rim of its caldera until sunrise.

Necroclasms gain a deflection bonus to their AC equal to their Charisma bonus. Additionally, during the day they gain a +4 profane bonus to their AC.