Worm of Minauros (CR 15)

Gargantuan Magical Beast
Alignment: Neutral
Initiative: -2(Dex); Senses: tremorsense 60 ft. and Listen +19

AC: 19 (-4 size, -2 Dex, +15 natural), touch 4, flat-footed 21
Hit Dice: 25d10+198 (335 hp)
Fort +21, Ref +12, Will +17
Speed: 20 ft., Burrow 20 ft., and Swim 10 ft.
Space: 20 ft./15 ft.
Base Attack +25; Grapple +49
Attack: Melee: bite +34 (2d8+12/19-20) and sting +32 (2d6+6 plus poison)
Special Attacks/Actions:
Attack Options: Awesome Blow, Cleave, Great Cleave, Imbroved Bull Rush. poison (injury, Fort DC 29, 1d6 Str/2d6 Str), Power Attack
Abilities: Str 36, Dex 6, Con 25, Int 1, Wis 8, Cha 8
Special Abilities: improved grab, swallow whole
Special Qualities:
Feats: Awesome Blow, Cleave, Great Cleave, Improved Bull Rush, Improved Critical (bite), Multiattack, Power Attack, Weapon Focus (bite), Weapon Focus (sting)
Skills: Listen +19 and Swim +20
Advancement: -
Climate/Terrain: Minauros (3rd level of Baator)
Organization: Solitary
Treasure/Possessions: -

Source: Fiendish Codex II

This slow-moving creature reacts with disinterest even when badly hurt.

Skills: The worm of Minauros has a +8 racial bonus on Swim checks to perform special actions or avoid hazards.

Improved Grab (Ex): To use this ability, the Worm of Minauros must hit an opponent of any size with a bite attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity. If it wins the grapple check, it establishes a hold and can try to swallow the opponent in the following round.

Swallow Whole (Ex): The Worm of Minauros can try to swallow a grabbed opponent smaller than itself by making a successful grapple check.

The swallowed creature takes 2d8+12 points of bludgeoning damage and 8 points of acid damage per round from the Worm of Minauros's gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal a total of 25 points of damage to the gizzard (AC 17). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.

The Worm of Minauros's gizzard can hold 2 Large, 8 Medium, 32 Small, 128 Tiny, or 512 Diminutive or smaller opponents.