The most powerful nightshade is fortunately the rarest as well: the nightwave, a sharklike terror 100 feet long. Thank goodness it stays underwater—oh wait, it can fly. And its swallow whole attack includes energy drain. To quote Apocalypse Now, “Never get out of the boat.” Better yet, don’t get near the water.
However, just because it’s CR 20, that doesn’t mean you have to wait until your party is nigh-epic level before you introduce the nightwave. It could be the subject of legends in your campaign long before the party is ready to fight it. And since so many of a nightwave’s powers are area effects—channel energy, unholy blight, and the phenomenal blackest depths ability come in particular—you can foreshadow the nightwave with a number of close calls over the course of several seafaring adventures…
The sailors of the Scarlet Isles speak of Hitam Alun, the Ebon Wave. Presumably a nightwave, this deadly creature never bothers to actually attack. Instead her very passage seems to summon the black depths of the ocean itself, crushing keels and snapping outriggers. Thus crippled, the vessels are easy targets for the lacedons, draugrs, and shadow rays that follow like remoras in her wake.
Adventurers contemplating assaulting a nightwave find they have unusual allies—a city of aboleths who are equally eager to not have to share their sea with an undead monstrosity. If the party allies with the aboleths, the psionic fish-monsters offer them invaluable magical aid in surviving the ocean depths and banishing nightshades. But these gifts are double-edged swords—every aboleth rune the party members learn to bind the nightwave in its trench also makes them more vulnerable to aboleth control…
The Night Thresher was once a noble cetaceal. A trip through a nightwave’s gullet changed that. Now a nightwave herself, the only vestige of her former existence is her preferred prey: vessels of a religious nature, especially those carrying missionaries, pilgrims, crusaders, and especially the treasure hauls of templars.
—Pathfinder Bestiary 2 202
I forgot to give a nod to syringesin and a *ba-du-bum-TSH* to Whelp for their thoughts on the night hag.