I'm not sure if this is confirmed or not, but if it's true it should be updated on the Biorr of the TwinFangs page. Basically I went through the entire game except for the final boss. As you know, you can still enter the other worlds before going back below the nexus. I decided at this point to lower my Character Tendency to start the Mephistopheles quest. So I start my killing spree, however I ran into an issue. When it came to killing Biorr I realized that I hadn't saved him yet, therefore he was not in the nexus. So I went to save him, thinking that even though the boss was already dead I could Evacuate and find him sitting next to the Valley of Defilement Archstone. As I made my return, no Biorr, much to my dismay. I load back into 1.3 and back track to find he is no longer in his cell. I evacuate again, no Biorr. So then I go through 1.3 and 1.4 (keep in mind I'm at End Game so I'm holding a TON of souls). All bosses and minibosses were killed beforehand, and Biorr was no where to be found.
This pretty much leaves me stuck at this point in the Mephistopheles mission. I hope I'm wrong, but if I'm right, others should be warned.