I've recently become in acquiring all of the dragon priest masks. I have 3 of them, before I cared or new that they were special in any way, I fought and killed Otar the Mad, I didn't take his mask but now I want it. I went back to Ravngald to see if his body was still there but I couldn't find it. Is there any other way to get the mask or a way to fight him again. Or is it just that I looked in the wrong spot to find him and need to continue looking.

It's gone now.

If you're on PC you can add it with a console command.