Nydus, the Black Sun, hangs high over Malstaris and spills heavy darkness across that plane. It does not keep a fixed height. Now and again, on no schedule anyone has ever charted, it sinks toward the floor of Malstaris. Because Malstaris is the underside of the material slab, when Nydus nears that floor it also nears the living world pressed against the other side. That descent is a Shadowrift. While it lasts, cold and dark bleed up through the ground into the Material Plane, and the wall that holds the realm of the dead apart from the living goes thin.
The dead rise on their own
The cold is the least of it. The true danger is what the dark does to the recently dead.
When a living thing dies, its shadow is pulled down to Malstaris by the current of the First Dark, the oldest of the shadows, who drains every released shadow out of the world. Where that current runs strong, a freed shadow sinks away and is gone. A Shadowrift floods the living world with so much of Nydus's darkness that the current stalls. Shadows that should fall to Malstaris pool in the dark instead, the way they pool year-round at the River of Wights, only now it happens across whole countries at once. A corpse still close to its own shadow needs no necromancer to knot the strand back in place. The dark holds it there for free. The dead stand up by themselves, in their hundreds, with no caster who paid for them in Deoric and no master to call them off.
This is the rift's particular cruelty. Necromancy is costly and answerable: someone bleeds to raise a host and keeps bleeding to hold it, so a host has an owner and a limit. A Shadowrift raises the dead the way the River of Wights does, but everywhere, and answers to nothing. When Nydus climbs back to its height the current resumes and the loose dead fall still as their shadows finally drain. Until then they are simply loose.
What crosses over
The thinned wall lets traffic move the other way as well. Things native to Malstaris, which were never alive on the Material Plane and hold neither soul nor spirit, cross up into the cold the rift makes for them. Umbral agents come first, the enforcers of the shadow-realm. Behind them come its larger hunters, the Devourers and Mind Fiends and Voids that keep their dens in Malstaris. None of them belong to the surface, and none stay once the rift closes. For as long as the Black Sun hangs low, they hunt the dark country it has opened, and they treat the living the way the living country treats vermin.
When the Black Sun comes down, bury deep and burn what you bury. The ground gives it back otherwise. — a saying kept along the Crimson Coast, where the dead rise without help even in ordinary years
The Dark Night
The worst Shadowrift on record is the one the Lost Ages remember as the Dark Night. From 12,000 to 11,500 BSD, Nydus hung so low for so long that the sun never returned at all, and the little that had been rebuilt after the False God's collapse came apart in five hundred years of unbroken dark.
That same descent is the probable cause of the Blight of Arcanus. The dark pouring up from Malstaris poisoned the Ezz that runs through the Psywinds, and because three of the world's four magics draw on the Ezz, all three turned lethal to their casters at once. The empire of the day had rebuilt on its elemental magic, the practice its own scholars miscalled the aetherial arts, and it fell the moment that power began killing the people who used it. Deoric, which pays in titan material and in the caster's own life rather than drawing on the Ezz, was hit differently. Its practitioners may have been the only ones who could still work magic at all, and survive it. The era buried its own memory in the Oblivion Years that came after, so no record names the rift as the killer outright. The catastrophe survives only in what it left behind: a five-hundred-year night, a dead empire, and a whole generation of the violently dead that no living hand had raised.
Five hundred years without a dawn. The snow that fell in the first decade never melted, and by the last the drifts stood higher than the dead cities they had buried. What moved on the ice moved slowly and did not need to see. Over all of it hung a sun that gave no light, only cold and the steady weight of its dark.