Would anyone be able to tell me what approximate area I could farm for a chance the mask would drop? Can’t seem to find anything searching Google or the forums.
Pretty much random. No specific area has a higher drop rate for it.
Epics like Herald’s Mask have no special drop zones.
All you can do is kill mobs in that level range (35ish), and hope for the best.
Play veteran for an increased number of Hero Monsters, if you can.
That´s about it.
Good luck. 
Heh, it dropped from the Lord Executioner’s glorious cache.
its random, pretty sure
It’s random but seems to drop fairly often. I think I’ve seen like 4 or 5