1) Prepare a separate monster cup for all the masks.
2) Each time a monster surge occurs, draw a random mask and put it in play on the surging gate. If a generic monster surge occurs as a result of a specific Mythos card(that is, with no gate opening location), a mask would emerge from every open gate on the board.
3)Masks are spawn monsters; they don't count against the monster limit, and are not removed from the board when a gate with their dimensional symbol is closed or sealed.
4) Each time an investigator defeats a mask, return it to the masks' cup, unless he can ignore their Endless ability. Any masks that are claimed as monster trophies can be spent as Clue tokens for the purpose of sealing gates and in final battle against Nyarlothotep.
5) Each time a mask is evaded , put it on Nyarlothotep's sheet along with a Clue token from the bank.
6) For each mask on Nyarlothotep's sheet, the total amount of Clue tokens required to seal a gate is increased by 1.
7) Should Nyarlothotep awaken, all mask monsters on its sheet must be defeated before proceeding to final battle.