Yyyyup. There was the usual spiel, submit to the darkness, blah blah blah, but the thing was...

His scent wasn't right either. I didn't know who he was... but he wasn't Ansem. I'd know Ansem's stink anywhere... and I wasn't smelling it on this guy.

Turned out, it was DiZ. He'd been disguising himself as Ansem at least part of the time I was coming up through the castle - prodding me along, so I'd fight. Fight the darkness, and fight Ansem.