The Swarmgate is a slight misnomer in that it was not a single gate, but an area where portals frequently appeared and disappeared, all bridging to the Swarm Plane. Because the Swarm Plane is a harmonic plane, the area was full of artificial mana crystals which could be harvested, though at great risk due to the creatures that would cross into this plane when the portals appeared.
The Swarmgate was located just east of Nevar.
Unaware of what would lie on the other side, the ravens created the Swarmgate two centuries ago in order to artificially cause the production of mana crystals. They were successful, but also unleashed the Swarm on themselves.
The Swarmgate was intentionally kept open by the Council of Magi in order to continue harvesting crystals until, in recent times, High Commander Falkstrom lead the Ravenguard into the Swarm Plane, where, against the Council’s wishes, they sacrificed themselves to close it using an eldritch ritual.