Sschindylryn's Gate

Toril-Oerth connecting portal leading to Erelhei-Cinlu in Greyhawk's Underdark.

It was discovered by scouts from the city of Sschindylryn. Sschindylryn is a drow city under Cormyr known for its use of portals to explore and conquer other parts of the Underdark. During a battle with Menzoberranzan, the portal was damaged, and none from the western city know how to operate it or to where it leads. (source - WotC web site)