Demon gate

There are four demon gates (or spirit gates) in the world, according to the elf Queen Xiiltharra, who disclosed the location and existence of the gates to both Sisren and the dragon rider Sela Matu in book 8, the Fall of Miklagard. 
The demon gates are portals to the spirit realm (the After Realm). One gate is located underneath Aonach Tower, the other in the Catacomb of Kings in Miklagard, and one is in the capital city of Morholt. The location of the fourth gate remains undisclosed.