Okay so there's this entry in the Setting section of the CRB:
In addition to searching for alien technology within the planet’s vaults—either on official contract or by slipping past drow defenses—visitors often come to Nightarch to study the eponymous arch at the center of the city. This arch is similar to those that link many of the system’s worlds—and a functioning ring of such portals within the city provides access to most of the other Pact Worlds—yet this particular arch is covered in markings that have so far defied all efforts at analysis. When it activates, spontaneously and at rare and random intervals, it creates a gate to unknown landscapes. So far, no group that’s passed through these short-lived portals has ever returned.
That sounds a lot like a non-sentient version of the Guardian of Forever from the Star Trek episode City on the Edge of Forever. Am I crazy? Do you think that was thrown in for the fun of it.
Either way, it seems cool to me.
That or a possessed Stargate.