But it wouldn't really be fair.
There should be a visual difference between Mask and an actual player so that the other players can tell which one is Mask.
Meet “Mask”; This entity will only appear in multiplayer lobbies. They will attempt to fool them by making themself look like one of the players. Touching this imposter will lead to instant death. It will also follow you for 3 doors until it disappears. If the player it impersonates perishes, then the entity will either instantly disappear, or it will impersonate another if there are 2 or more people left. Quite an annoying entity, is it not? Well it can only appear on doors 50 - 70