I was just thinking about it and I actually had a hard time buying that a being can be a Dragon Priest and the Dragons don't notice he is a Dragonborn. This Dragonborn would have been alive at the time of Alduin so I would have thought that Alduin would have sensed the fact that the priest had Dragon blood. Maybe I'm overthinking it but it just strikes me as wierd.

Maybe that's why he turned against the dragons, when Alduin discovered his potential power.