(Illustration by Wayne England comes from the Paizo Blog and is © Paizo Publishing.)
Sea dragons are blustery but basically good-hearted commodores, forever wandering across the vast domains they protect. Krakens are slave takers and empire builders, grasping for more power above and especially below the waves. Sea serpents are practically natural disasters, violent irruptions (it's a word) that shatter a captain’s calm, control, and keel.
But when you want the classic sea-monster-that-holds-a-city-hostage, that’s when you turn to the cetus.
This is the monster that Perseus fought, and I admire Bestiary 5 for being willing to go to the mat to make this monster live up to its mythological rep (even if there are no mythic ranks in its stat block). It’s Colossal in size. It regenerates damage. It can control water, winds, and weather, allowing it to box opponents into their own harbors and foil aerial attacks. Speaking of aerial attacks, it can leap 1,200(!) feet in the air to lunge at flying opponents…and even if the cetus’s victims are not unlucky enough to get fast-swallowed whole, the cetus’s dispelling bite is bound to ruin many of their enchantments (including possibly the ones keeping them aloft). Heck, the cetus is even just plain bad luck—literally just being in its vicinity can be enough to screw up your dice rolls for the next minute (an eternity in combat).
In other words, this is a beast truly out of legend. Normally I don’t like monsters that seem specifically designed to foil PC (and player) actions and drain their spell reserves (I’m looking at you, golems). But for the hostage-taking, sacrifice-devouring, city-extorting cetus, it feels right. The designers even throw the players a bone straight from the Perseus myth—the cetus is vulnerable to petrification. So the next time your adventurers are at the flea market, keep an eye out for pickled medusa head…you know, just in case.
Petrified does not mean dead. Adventurers race to stop a locathah terrorist from resurrecting a legendary cetus, currently lying like a stony statue at the bottom of the Devilfish Deeps.
What’s more terrifying than a cetus? Any being powerful enough to use a cetus as a mount. A greater dullahan antipaladin rides a cetus into the mouth of Hellbone Harbor, bringing dark tidings from below. The cetus also bears a howdah containing all the souls of the dullahan's many, many victims.
The Afterlife is a river—one that flows every onward toward Oblivion. Even what mortals conceive of as the Four Blessed Heavens or the Thirteen Precincts of Hell are merely ports of call along the river’s course. But one rule of the Afterlife is ironclad: No vessel may travel upstream. Individuals may sometimes escape the River of Death through powerful magic, fell bargains, or even dogged, determined fording upriver (usually resulting in undeath by the time the pass back into the mortal world). But any attempt to build and sail a ship upstream is met with a fiendish cetus determined to crush the blasphemous vessel and all aboard.
—Pathfinder Bestiary 5 54
Speaking as someone whose eldritch knight nearly single-handedly took out an ulgurstasa courtesy of fireballing while flying, I can tell you the cetus’s Impossible Leap (Su) ability is terrifying.
One fact I didn't tackle above is that the cetus is technically a dragon. I’m a huge fan of one-off dragons, and I like the idea of one of these crashing an otherwise stately gathering of metallic, chromatics, and imperials…
Have I mentioned yet how much I enjoyed the Pathfinder setting sourcebook Distant Shores? In that book the mythic hero-gods of Aelyosos have a thalassic behemoth problem, but in your campaign maybe a cetus would do the trick instead.
Also, I’m repeating myself from my last entry, but the cetus is an excellent monster for a Scarred Lands campaign.