An opening to Quor'toth, the darkest of the dark worlds.
Quor'toth, the darkest of the dark worlds, was a hell dimension that can be accessed only through tearing the very fabric of reality and not via a regular portal, like Pylea or other dimensions.
Sahjhan opened the portal (although not technically a portal, as the only way to get to Quor'toth is to punch a hole through reality), thus forcing Daniel Holtz to jump into the portal with baby Connor. There, Holtz raised Connor for sixteen years, as time moved faster than on Earth, teaching him how to fight and hunt and instilling in him a deep hatred for Angel.
Sluks, demons indigenous to Quor'toth, were what led Connor to find the fissure out of the dimension.