This enormous wolf-headed sea serpent's maw leads into what seems to be an endless expanse within.
Ketesthius CR 13
LE Gargantuan magical beast (aquatic)
Init +11; Senses darkvision 60 ft., low-light vision; Perception +14
AC 28, touch 13, flat-footed 24 (+7 Dex, +15 natural, -4 size)
hp 200 (16d10+112); regeneration 10 (fire or acid)
Fort +17, Ref +17, Will +10
Speed 20 ft., swim 40 ft.
Melee bite +22 (4d8+10/19-20 plus grab), 2 claws +22 (2d6+10/19-20)
Space 20 ft.; Reach 15 ft. (20 ft. with bite)
Special Attacks bigger inside, fast swallow, grab (Gargantuan), swallow whole (see text)
Spell-Like Abilities (CL 13; concentration +13)
Str 30, Dex 25, Con 24, Int 5, Wis 16, Cha 11
Base Atk +16; CMB +30 (+34 grapple); CMD 47
Skills Perception +14, Swim +29
Languages Common (can't speak)
Environment any oceans or coastlines
Bigger Inside (Su) A ketesthius's stomach is an extradimensional space, much larger on the inside than its physical size would allow. A ketesthius can swallow any Gargantuan or smaller creature, and its cavernous stomach is an entirely new location, its size varying wildly from ketesthius to ketesthius. Creatures swallowed by the ketesthius are not considered grappled, nor do they take damage each round, and damage to the walls of the stomach do not harm the ketesthius or allow the creatures to escape. Creatures trapped inside can generally escape only via teleportation magic that crosses planes or if the ketesthius is defeated or nauseated from outside. As a result of its unusual anatomy and huge appetite, a ketesthius often has other monsters living inside its stomach, which in turn feed on things the ketesthius swallows. A ketesthius usually has from one to four monsters of CR 8-9 in its gullet. Sea creatures like dire sharks, dire crocodiles, giant octopus, or giant squid are most common, but less frequently giants or other land-dwelling creatures live in its gut as well. Ketesthiuses are even known to swallow each other, though only on rare occasions.
Ketesthiuses are large sea monsters with wolflike heads and foreclaws, eel-like bodies, and powerful fish tails. They are voracious eaters with great jaws that can gulp down prey that can be as large as themselves. Creatures a ketesthius swallows are not digested immediately, because ketesthiuses' vast stomachs and unique anatomy allow them to sustain living creatures inside their gullets indefinitely. Entire ecologies can form within a single ketesthius's extradimensional digestive tract. Though the stomach's ecosystem is capable of supporting life, it's far from a welcome environment: no sunlight comes in, the air is thin and dank, and most creatures trapped within become feral in short order. Larger creatures that end up inside a ketesthius have a difficult time surviving long term, and must resort to quickly snapping up prey that their host later swallows.
Though it makes no attempt to hoard treasure, within a ketesthius's vast gut lies all the treasure of the creatures—and even ships—that the ketesthius has swallowed. More than once, tales of lost treasure within have led foolish adventurers to risk being intentionally swallowed by a ketesthius.
An adult ketesthius typically measures over 50 feet long. Its weight varies based on the size of its extradimensional gullet, but usually ranges from 5-60 tons.