|Type|| Magical beast|
|Environment||Any cold (moon or outer space)|
Source: Bestiary 6, pg(s). 174-175
Yarthoon is an immense pale blue worm with a many-toothed maw opening amid a ring of glowing eyes. Her exhalations fill the region with freezing mist. She is 250 feet long from head to tail and weighs 14,000 tons.
Yarthoon can exist comfortably in outer space and dwells on the frozen moon of Golarion. The planet orbited by the moon is a constant fascination for her, and certain events have been known to specifically attract Yarthoon's attention. Their exact nature varies, and even the most learned scholars argue over what is the correct cause.
Unlike many kaiju, Yarthoon never seems to purposefully seek out civilisations to destroy, but still wreaks havoc incidentally. Her visits to Golarion typically last for only a few days before she returns to her home on the moon.
Yarthoon has repeatedly returned after suffering what seemed to be a complete defeat. Certain kaiju scholars have speculated that Yarthoon might not be a single creature, while others think that Yarthoon is but a larval kaiju, and that while only one may live at any time, numerous eggs lie in a hidden crèche on the moon, ready to hatch a replacement as needed.
Yarthoon often clashes with Mogaru when she is attracted by the devastation wreaked by Mogaru, or when Mogaru senses her presence and investigates when she comes to visit. Both are unwilling to finish fights and the winner is content to let the loser flee. Both despise Lord Varklops, and have teamed up several times to drive him off.