Explain the Nadir of Shyish to me?

I know how it was created and have a general idea of what it is? It was caused by the Skaven interfering with the Necroquake and basically causing a magical blackhole to form in the heart of the underworld. But what is it in more detail? Can it be stopped? Is Shyish doomed to slowly be devoured by it? What's Nagash doing to stop it, if anything? Just anything to clear up the confusion would be real helpful.