It was obviously Moridin as he is the only one that can channel the True Power and Rand didn't fell saidin or saidar when the person channeled. As I believe Moridin is Beidomon this would also explain how Rands rescuer was able to give him advice concerning Sammael's personality- "He always liked destroying a man in sight of one of that man's triumphs"(p 635). This is because Beidomon/Moridin would have known Sammael both before and after the breaking. I believe Beidomon has his own agenda and is not merely a follower of the DO. He probably changed his name to Moridin (Death) when he saw the death and destruction he caused when he released the DO.