Planescape Canonical Campaign
The latest in a long line of former Gate Towns from the Outlands. When a gate town becomes similar enough in belief to the plane it's portal links to, the entire town will shift to that plane and another town on the Outlands will become the new gate town. Some gate towns strive for this, and others – like the gate towns to Baator resist it as much as possible. In all cases, however, it is seen as a victory for the plane that expands its territory and influence.
Darkspine lies largely in ruins, but does have a small population struggling for survival here. They hang on primarily due to it's former portal to Baator now being a portal to the current gate town of Ribcage. Furthermore, Baator is by design highly restrictive in portals to and from the plane. The majority of (legitimate) traffic that comes into and out of Baator travels either on the River Styx or through the Ribcage-Darkspine portal.
Those who live within what remains of the town serve travelers passing through. They provide the writs of passage that protect travelers from roaming patrols of Baatezu searching for invaders (and fodder to conscript into their armies for the Blood War). Of course, the amount of protection a writ of passage offers varies greatly depending on the deal one can make with the administrators. Furthermore, the standard services of (what passes for) inns, supply shops, taverns, and the like are present, although are typically in partially collapsed buildings and rarely last long. Mere suspicion of a spy will lead to a disastrous raid by the Baatezu who have no concerns for collateral damage against the establishment.
Lastly, some rebels work within Darkspine in attempts to save it. With enough effort, it can be shifted back to the Outlands although few hold out hope for that. At the very least, the longer they can help prevent Ribcage from slipping, the longer it can maintain Darkpsine's status as a major entry port to Baator. For once Ribcage slips into Baator, Dakrspine's portal will close and the town will no longer serve any useful purpose to the Baatezu. The residents of Darkspine know that would lead to a swift and brutal end to the meager semblance of life tey maintain.