I am aware there are 8 dragon priests with masks, but in Silverdrift Lair I just killed a dragon priest and he had no mask on him, are there more than 8 dragon priest perhaps that have no masks or is this another bug?, I hope it's the former, I'm two away from completing the collection!

i think there are several unnamed dragon priests without masks

Thank you, that's good news, I can breath again!

there are other dragon priests... they are just called dragon priests and don't have masks... u should come in contact with them a lot after level 40...

They are the equivalent of liches for skyrim.