Rifts
Tears through to the Beyond, torn open by the older spirit-calling magic of the Barren Lands
A Rift is a tear: a way through to the Beyond, and not a thing anyone wants standing open near them. Magic that calls on spirits opens one, which is why work of that kind is so widely distrusted.
What Idir has of them came by way of the Barren Lands, where they teach an older kind of magic than the Congress will license, and by way of Yara, who trained there. When she gave the party a rune-etched bone that would let them put questions to a corpse, the warning came with it: magic of that kind tears Rifts (S2E18). She said later that it should never be worked near a curse-keep, because it might make matters worse.
How wide one opens, what uses it, and whether it can be closed again were never written down. The bone has been broken once since, over a dead lieutenant, and no Rift came of it.