Plague whales are a type of whale known for carrying pestilence between distant lands. While some are created intentionally by necromancers, most seem to form naturally by some unknown force in the Mhasmuth Ocean where they are endemic. They pose a constant threat both to sea-going travelers between the Three Great Seas and the Spine and to the coastal communities of the Spine where they wash ashore.
Plague whales are undead formed from living whales as their flesh and organs necrotize and rot from within. As they begin to transform, plague whales at first continue their existence as normal but their behavior gradually changes as the necrosis reaches their nervous system and brain, at which point they start to act erratically and with violence. Drawn upwards and to the light, the whales target vessels or creatures on the surface of the ocean in particular. Even if a passing ship survives the thrashing of the whale they are not necessarily safe, for in the violence the diseases carried by the whale can be unleashed and begin to spread. The whales are dangerous long before this point, however, as even in the earlier stages of their necrosis they harbor pestilence - many an unwary whaler has harpooned a plague whale and only later identified what they caught.
Even in the last stages of their decay plague whales still appear largely as they do in life, their intact skin hiding the bloated, rotting tissue within. Beached plague whales can explode spectacularly if disturbed, spreading rotten flesh and disease to quite some distance.
Any type of whale can become a plague whale. The natural method of their creation is not precisely known, but it is believed by many scholars that some disease-carrying entity lurks in the darkness of the lower depths of the Mhasmuth Ocean, where it infects, intentionally or otherwise, passing sea life.