I was thinking that since ME said zilch about the Immortal we are free to speculate. He could be a Highlander (I'd say Methos but he tended to remain in deep cover) or Conrad Nomikos from Roger Zelazny's This Immortal or some other skiffy character. It occurred to me that if a Buffy fanfic writer wanted to use him, then the background given by ST could flesh him out (though how he knows he died in 3834 BC causes me to doubt his veracity). I actually can picture Akharin (a younger James Daly) doing what he did to Angelus and Spike and satisfying Dru and Darla.
Wonder if we'll learn anything if/when those BtVS comics come out.

Shakatany