The Gate of Darkness is a massive sealed door underneath Cyrum castle. Its key is Roan's medal. The door itself appears to be very similar in design to the Granas seals. Behind the door is an Automata factory, which was used by Valmar to produce his soldiers. There is also a Granas seal inside, which held the Claws of Valmar. The door is opened by the King under the misconception that its opening will bring prosperity to his kingdom. He was fooled by Melfice. The factory was destroyed essentially sealing the gate for good.
The Gate required a guardian to be posted at all times in front of. To allow no one to open it, but also to allow nothing out. When King Roan visits the gate, the guardian is Menory. He is killed when the Gate is opened.