With the revelation that demons only die in Nether or heavy fel places like Nathreza / Argus. This is why Uther says the Lich King can only be destroyed in close proximity to the Frozen Throne aka his Nether Anchor point.Ner'zhul's spirit was placed within a specially crafted block of diamond-hard ice gathered from the far reaches of the Twisting Nether.
This also lends more credit to naysayers about Ner'zhul's spirit being destroyed.