My guess is that Orin is a chosen of Bhaal and the Dark Urge a) the child of Viekang b) child of Charname c) someone who absorbed the Bhaal essence polluting the waters of the Boareskyr bridge
Nah... I still think that Tav could be the child of Charname and Durge might just be an artificially created Bhaalspawn, remember Charname wasn't all murdery like Orin or Durge.
I also don't think Orin and Durge are the same person.
