The Rift was caused by Dunkelzahn, yes. In a moment of extreme duress (i.e. he discovered that the Horrors were getting help and were about two days from bridging the gap between their world and ours instead of, you know, a thousand years) he self-immolated. Certain individuals (*cough* Harlequin *cough*) believe it was a nuke that did the job; I highly doubt it. Very high level, possibly a unique spell derived from what Dunkelzahn knew of his brother Ghostwalker's ability to, ahh, 'planewalk', yes. Nuclear, though, no.