I don't understand why ESO doesn't write down which dungeon masks drop from. I had to Google each mask name individually and figure out which dungeon they drop from.
Please let me know if these are right/wrong.
Euphotic Gatekeeper September 4-11 Graven Deep
Nazaray September 8-15 Shipwrights
Molag Kena September 18-25 White-Gold Tower
Llambris September 24 - Oct 2 Crypt of Hearts
Balorgh September 25 - Oct 2 March of Sacrifices
Selene September 29 - Oct 6 Selene's Web
I've asked around, and it seems veteran ESO players memorize all this information for each dungeon in the game. So ESO doesn't bother writing out which mask drops from each dungeon for new/casual players. Hopefully this helps other confused players trying to figure out which dungeons have an RNG chance to drop masks.