Like it says on the tin.
Our scientists, while studying String Theory, open a portal through which comes not the Mist, but RUBY EYES SHABRANIGDO! Yes, its even worse than the Mist because he doesn't come in pieces; Shabby appears as a complete entity, as he was in his fight against Celpheed!
The Mazoku Lords follow, and so-on, so-forth.
Until the point where Naga clones begin to appear, causing mass suicides and some redhead madwoman begins blowing up entire cities with Dragon Slaves.
We have become occupied by the Slayers Universe.
Can we survive?