Aha! I'm curious, for I have SPOILERjust beaten Quelaag with the help of some co-op ghosts. Rang the second bell and descended into the Demon Ruins for a quick look, but turned back cause that place is creepy. I'm back in the swamp now and was planning to make my way to the massive gate the bells have opened.
Also, I got invaded and managed to defend myself successfully for the first time. I actually owned the guy (who was attacking with a heavy two-handed weapon) by just moving around him and stabbing him in the back twice. The battle only lasted a few seconds, which was a pretty epic win moment : )