This one:
Tarrasque
Colossal Magical Beast
Hit Dice: 48d10+576 (840 hp)
Initiative: +7 (+3 Dex, +4 Improved Initiative)
Speed: 20 ft.
AC: 35 (-8 size, +3 Dex, +30 natural)
Attacks: Bite +57 melee, 2 horns +52 melee, 2 claws +52 melee, tail slap +52 melee
Damage: Bite 4d8+17, horn 1d10+8, claw 1d12+8, tail slap 3d8+8
Face/Reach: 40 ft. by 40 ft./25 ft.
Special Attacks: Frightful presence, rush, improved grab, swallow whole, augmented criticals
Special Qualities: Damage reduction 25/+5, carapace, immunities, regeneration 40, scent, SR 32
Saves: Fort +38, Ref +29, Will +20
Abilities: Str 45, Dex 16, Con 35, Int 3, Wis 14, Cha 14
Skills: Listen +21, Spot +21
Feats: Blind-Fight, Combat Reflexes, Dodge, Improved Initiative, Iron Will
Climate/Terrain: Any land
Organization: Solitary
Challenge Level: 20
Treasure: None
Alignment: Always neutral
Advancement: 49+ HD (Colossal)
Combat
Frightful Presence (Su): The tarrasque can inspire terror by charging or attacking. Affected creatures must succeed at a Will save (DC 26) or become shaken, remaining shaken until they leave the area of effect.
Rush (Ex): Once per minute, the normally slow-moving tarrasque can move at a speed of 150 feet.
Improved Grab (Ex): To use this ability, the tarrasque must hit a Huge or smaller opponent with its bite attack. If it gets a hold, it can try to swallow the foe.
Swallow Whole (Ex): The tarrasque can try to swallow a grabbed opponent of Huge or smaller size by making a successful grapple check. Once inside, the opponent takes 2d8+8 points of crushing damage plus 2d8+6 points of acid damage per round from the tarrasque’s digestive juices. A swallowed creature can cut its way out by dealing 50 points of damage to the tarrasque’s digestive tract (AC 20). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out. The tarrasque’s gullet can hold two Huge, four Large, eight Medium-size, or sixteen Small or smaller creatures.
Augmented Criticals (Ex): The tarrasque threatens a critical hit on a natural attack roll of 18-20, dealing triple damage on a successful critical hit.
Carapace (Ex): The tarrasque’s armorlike carapace is exceptionally tough and highly reflective, deflecting all rays, lines, cones, and even magic missile spells. There is a 30% chance of reflecting any such effect back at the caster; otherwise, it is merely negated. Check for reflection before rolling to overcome the creature’s spell resistance.
Immunities (Ex): The tarrasque has fire, poison, and disease immunity.
Regeneration (Ex): No form of attack deals normal damage to the tarrasque. The tarrasque regenerates even if disintegrated or slain with death magic: These attack forms merely reduce it to -10 hit points. It is immune to effects that produce incurable or bleeding wounds, such as a sword of wounding, mummy rot, or a clay golem’s wound ability. The tarrasque can be permanently slain only by reducing it to -30 hit points and using a wish or miracle spell to keep it dead. If the tarrasque loses a limb or body part, the lost portion regrows in 1d6 minutes (the detached piece dies and decays normally). The creature can reattach the severed member instantly by holding it to the stump.
Skills: The tarrasque receives a +8 racial bonus to Listen and Spot checks.
Monsters you would like to see in BG. 
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