The Nightmare

The Nightmare is a blackness, spreading from plane to plane, warping its nature, spreading monsters, and in the end consuming it.
The arrival of The Nightmare and spread of darkness is known as The Gloaming, while the final end of a world is sometimes known as Nightfall, but more commonly just The End.
The Nightmare spreads more easily along inter-planar connections, whether physical portals or simply the connection between a deity and its worshippers.
Though the extraplanar powers seem to have been able to contain the outbreaks of The Nightmare on the outer planes, they seem to have little interest in inviting more incursions. As such they have taken to cutting connection to planes infected by The Nightmare as soon they learn of the infection, leaving any worshippers left behind to their own devices.
This is not as readily enforced on the chaotic-aligned planes, where demons and proteans strike deals with infested planes despite the dangers. As such their planes are pockmarked with contained and uncontained infestations of The Nightmare, threatening to spread to connected outer planes.
The Old Lands, from which the city originates, were consumed by The Nightmare some 25 years ago.