The Cataclysm

The Cataclysm
Something goes wrong in Karros. What exactly has never been determined. But whatever it was causes a massive explosion, causing a massive swath of Karros to collapse into a mile-plus deep pit and a plume of smoke and ash to billow into the sky. A portal of sorts forms in the rift, which becomes known as the Hellgate, which allows fiends to enter the material plane unchecked. The Karrossians flee the immediate area.