The Daily Bestiary has always been setting-independent. So what does a unique flying cousin of the tarrasque, one of the spawn of Rovagug, do when we remove it from Golarion?
Answer: Anything it wants.
In your campaign Volnagur, the End-Singer, might be a creation of the Old Ones, demonspawn, or an abomination (unwanted progeny of the gods) similar to the 3.0 Epic Level Handbook’s chichimec. No matter what the origin story, it’s a literally unkillable, apocalypse-level monster likely to be the ultimate (or at least penultimate) encounter in your campaign. But the PCs have to stand up to it anyway. Thanks to the madness the End-Singer inspires, blood will run in the streets and nations will fall if it is not stopped.
Besides, won’t this be fun to say to players:
“You’ve heard of the tarrasque? This is like that. But flying.”
Burned by previous liaisons, yet needing a lieutenant, the demon lord Pazuzu once tried to breed via parthenogenesis. It did not go well. The resulting creature is not even a demon, just a colossal flying engine of destruction that Pazuzu’s own servants fear due to its sonic eye rays. Desperate to rid the Abyssal skies of the thing, Pazuzu will often lure the creature to the Material Plane to fly amok. But it always finds its way back to the Abyss—and its father.
Minotaur astronomers spot a meteor streaking for the city of Tinapolis. Disorder and lawlessness reign after their pronouncement. But when the object streaks into the atmosphere, it does not strike. Rather, the heat of entry causes the “meteor” to awaken from hibernation, unfurl its wings, and begin a carving a path of destruction with winds of vengeance.
Bardic masterpieces can go wrong. When you tap into the true magical harmonies, be they songlines or the music of the spheres, sometimes the tune plays you, not the reverse. Camiel Vascar may have prepared for years for his performance at the Oracle of Mielal, but he didn’t understand the real stakes. When the song took him, discordance took over, and it poured out of him until his body swelled, split, burst into light, and then hatched forth the End-Singer to ravage the world.
—Inner Sea Bestiary 48
Jason Nelson created Volnagur, by the way.