I'd say it will be extremely hard for the other PLAYERS not to know or notice.
Wearing the mask will prevent wearing helmets, group heals might do damage, the healing from poison etc.
On the other hand it should be doable to hide one's true identity from the other CHARACTERS.
What players/characters can or can't do is completely up to the GM.
Being able to set "undead"as a (permanent) status on characters would be an awesome tool to have.
