Fridays are turning into Whale Days here at The Daily Bestiary.
…DEATH WHALE DAYS.
Whaling is a brutal hunt in the best of circumstances, over many bloody leagues of ocean. It is no wonder then, given whales’ innate intelligence and the pain of the harpoon and the chase, that many of them rise as undead creatures. Sadly, they don’t confine their hatred just to those who wronged them, but to any vessel or port that reminds them of their pain. Worse yet, their flesh is infectious, turning their parasites necrotic and gathering scores of undead fish and birds in their wake. A bakekujira is not just an undead abomination; it is an unnatural disaster.
An adventurer finds a diary that appears to belong to one of her family members. The handwritten journal gives an account of the relative’s years on a whaling vessel. The relative appears to be hunting the same bakekujira the adventurer is hunting now. Stranger still, the many descriptions of ship designs and gunnery imply that the diary comes from the future.
A bakekujira destroyed the fishing village of Rabbit Foot Bay, crushing the boats and smashing every pier to splinters. Now, as if uncertain as to what to do, it has beached itself in the rice paddies outside of town, where it spends its time singing haunting melodies. The local governor wants the undead whale destroyed before it levels another port—or, worse yet, travels inland on the Shogun’s Road. The priest caste, on the other hand, is convinced that violence will only beget more violence. They, too, seek to hire adventurers who will bring the whale peace, either by a musical exorcism or by leading it to seas far from the shipping lanes of man.
The whales of the great voids, the astral leviathan and the oma (see Distant Worlds), have their own bakekujiras. Most of these are the results of hunting by astral and stellar whalers. But necrotic pulses from the coal-shrouded planets of the Night Brethren are known to corrupt whole pods of oma, who then rage toward the nearest trade route or starport. The astral variety are usually found singly, but are often trailed by recently minted devourers.
—Pathfinder Adventure Path #59 84–85 & Pathfinder Bestiary 4 15
I didn't mention it above, but the folklore of the bakekujira is worth the read. Check it out!