All drakes are degenerate. (The Bestiary series is a bit judge-y on the subject of dragon evolution.) But spire drakes…they’re just wrong. Because anything whose breath and bite are so corrupted they can breed wights simply shouldn’t exist as a natural creature. Yet the spire drake does.
My best guess is that some stunted offshoot of the umbral dragon line produced both shadow drakes and spire drakes, with shadow drakes getting a version of their progenitors’ shadow powers and spire drakes getting the umbral dragons’ taint of undeath. Or you can skip the evolutionary root cause and blame the spire drakes’ environment. Lands tainted by long exposure to undeath, storms of wild magic, or blood magic and other dark rites could certainly give rise to spire drakes. (Russ Taylor’s “Ecology of the Drake,” from Pathfinder Adventure Path #92: The Hill Giant’s Pledge, blames mutations in other drake bloodlines caused by the Golarion setting’s deadly Mana Wastes.)
The point is, spire drakes straddle the line between the natural world and worlds far, far worse. If PCs don’t want to find out what those worlds are like, they would do well to stay sharp and avoid the spire drake’s coup de grace.
Sometimes the hardest part of the dungeon is the journey home. When adventurers reach sunlight for the first time after plundering the Spiral Crypt of Dorn, they are met by a rampage of spire drakes demanding tribute—namely, all the adventurers’ magic items. If the adventurers refuse, the spire drakes attack immediately. If driven off, they will nonetheless attempt to harry the adventurers all the way back to civilization in a series of guerrilla attacks, hoping to make off with what magic and hunks of meat they can. Should the adventurers make it to the safety of the city walls, the drakes slink away in frustration…but not before warning an osyluth ally, in hopes that his infernal wiles may succeed where theirs did not.
In the Scar, divine magic withers and arcane magic blooms into wild energies. Spire drakes, horribly warped goblinoids (including the mudlike murds), and greedy oozes hunt in the shadows of the canyon that gives the Scar its name. Two of the most terrifying hunters in these lands are the spellscarred fext (a generic version of Inner Sea Bestiary’s Spellscar fext) Simon Kraal and his spire drake steed Lash. Though Lash cannot bear Simon aloft, she can rampage along the blasted landscape with the undead knight astride her back, and together they hunt down any magic-employing prey they can find.
There are no undead on Vashon. No undead that is, until they begin being born in the wake of spire drake attacks. Where the drakes come from is still a mystery, but the spawning wights they leave in their wake inspire horror in every Vashon man, woman, and child. Worse still, while these wights rarely last for long, other undead species seemingly unrelated to spire drakes have begun manifesting too, as if the door to the Realm of the Dead has suddenly been propped open. If there is one blessing in all these dark tidings, it’s that clerics and other divine casters are discovering new powers they can use against these monsters—spells and channel abilities that have lain dormant for centuries. But clearly these drakes must be exterminated and the way to Death shut again if Vashon is to endure.
—Pathfinder Bestiary 4 81