Despite being figures of legend, Scylla and Charybdis have never really been featured in role-playing—perhaps because the cursed Scylla was too singular and Charybdis too nebulously described. But the Bestiary series loves its mythical monsters and gifts the pair with pretty savage CRs. To my mind, the charybdis is best used to give some color to your oceans, especially as one of several bad options. For instance, a certain strait could be the PCs’ fastest route…if they dare face the rending claws of the beast.
The Lorelei of Ulster, a mermaid bard/enchantress of surpassing power, sings to lure sailors to their deaths in the maw of her pet charybdis.
The blockade of Marne is crippling the war effort of Terre D’Héron. But one strait too dangerous to be watched by ships of the line, is left unguarded. If a shipment of magical scrolls and weapons could be smuggled past the charybdis there, the Heron Throne would be saved.
Sages assure the superstitious that the charybdis is a simple monster, not the avatar of a vengeful deity. But when the people of Crooked Harbor abandon their traditional blood sacrifices, they are rewarded by a charybdis arriving to haunt the port's narrow exit to the sea.
—Pathfinder Bestiary 2 56