The ketesthius is the classic wolf-headed sea serpent you find scrawled in the corners of old maps. Since Pathfinder already has plenty of sea serpent-y beasts, this one clearly needed a hook…and boy did it get one: an extradimensional stomach so big entire ecosystems can live in there!
Of course, now that I think of it, I’m kind of surprised that we don’t have more monsters with magic/livable stomachs. It’s not exactly a fantasy trope (aside from maybe Pinocchio), but as a cartoon trope it’s one of the most common and enduring ones there is. (Seriously, name me a cartoon character who hasn’t being carried inside a whale, air whale, space whale, or a whale-like robot/starship at least once.)
If you're a GM, you’re probably way more excited by what you can put inside the ketesthius’s stomach than you are about anything else in its stat block. Hell, I’d be tempted to waive the bite damage if it gave me an excuse to dump PCs inside. Who cares about a +22 bite/claw/claw routine when you can make a dungeon out of intestines?
(Also, how does such a slow-acting, practically benevolent digestive system even work? It must operate on the thousand-year scale of a sarlacc. Actually, the most likely answer is that the ketesthius is essentially a carrion or filter feeder, with the live creatures in its stomach doing it the courtesy of either consuming each other or expiring from starvation, after which the resulting nutrients can be absorbed at leisure.)
Then again, CR 13 is nothing to sneeze at. When you need a shipping lane bottled up or a coastal city tormented, the ketesthius is compelling terror, even setting aside its magical gullet. If I were forced to compare, I’d say Bestiary 5’s cetus is probably the better combatant, good as the penultimate or final challenge of a given adventure, while the ketesthius is a better side trek encounter on a long sea voyage, or even an adventure opener if its stomach leads somewhere particularly interesting.
Also, given the ketesthius’s high CR, the average party who interacts with one will almost certainly be packing extradimensional gear of their own. It’s up to you whether this is a problem you hand-wave away, a potential escape route for magic-poor parties, or an excuse to recreate the end of Jaws…with your PCs and their bags of holding standing in for the oxygen tank.
Adventurers hunt a ravenous ketesthius that has recently disrupted shipping up and down the Saffron Coast. The creature shows the marks of having fought with a giant cephalopod, which may indicate a kraken drove it this close to shore…or is even directing the wolf-serpent’s actions. Any adventurers who wind up inside the ketesthius find half-chewed tentacles to support this theory…as well as an aged dragon turtle, nearly albino-white after centuries of slow digestion, who is desperate for either freedom or the sweet release of death.
Engulfed en masse by a ketesthius, an already debased deep merfolk tribe has struggled to both survive and hold on to its identity inside the creature’s extradimensional stomach. Breeding is carefully regulated, a tradition of ritual sacrifice has morphed into ritualistic cannibalism, and the tribe’s mingled reverence and fear of the Old Ones has expanded to include their gluttonous captor. Adventurers who find themselves in the merfolks’ domain will likely have to fight their way out (and out of the ketesthius as well). Before they do, though, they may pick up some useful information, as the deep merfolk have preserved lore and rites pertaining to the Old Ones that no land-dweller recalls.
Reports on the landscape inside a ketesthius’s stomach vary from account to account. Some describe, as one would expect, dark seas within cramped rubbery chambers, islands of flesh peopled by desperate victims, or even bioluminescent coral reefs. Yet more fanciful tales exist of wolf-serpent stomachs that hide a sleeping ocean giant’s bridal chamber; a fey stone circle complete with centaur seer; a demiplane of sliding cubic chambers peopled by mites, pixies, and worms that walk in bishop’s regalia; and at least two gates to planar realms: one to Pandemoniak, the other to the Violet City of Loss.
—Pathfinder Bestiary 5 150
The Pathfinder team had to dig—or get playful with Greek, I’m assuming—to come up with the ketesthius’s name, because Google is giving me nothing.
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