Ghostwalker tapped into the power of the four artifacts, which allowed him to close the rift. In fact, the purpose of the four artifacts may have been to open and close metaplanar rifts. The trick, of course, is knowing how to get the specific metaplane you want. Since Ghostwalker took the artifacts with him, I don't doubt he'll be back soon.
As for where he went, it might be he had some unfinished business on the plane where he was travelling during the low time. Or perhaps he intends to bring Dunkelzhan's spirit back from the bridge. Or perhaps he's found some way to weaken Verjigorm. Who knows?
