Plane gate leading from the Prime Material to Kareth, a plane in the Outlands.
1601
The gate needs to be annointed with royal blood before it will open. As a result, it hadn't been used for centuries and the Erlyid faced it in marble and used it as a triumphal arch.
A conspiracy of vampires and their thralls plotted to open it and release Cain the First Vampire, but were foiled by a group of adventurers.