So according to the Ebony Throne that Dane has, this otherworld place that consumed london and corrupted the x-men in this issue is all because excalibur was pulled from the stone by Arthur. Excalibur was the seal that kept the otherworld at bay but because merlin didn't want arthur to put the stone back he created the seal that protected the planet from outside magical forces and tied that spell into being sorcerer supreme. So when strange died, the barrier fell and otherworld started bleeding through. Essentially everything that's been happening is Merlins fault also Danes daughter is a Mutant but she doesn't know yet :hestonpls