Involvement
Like the other Masks, the Mask of Vengeance is initially hostile by virtue of wearing a Whispering Mask, placing her under the effect of .
If the party manages to encounter her after Ethel is dead, she will no longer be hostile, but her sanity will no longer be intact. She will still be hostile if Ethel has been defeated but the party accepted the offer of Hag's Hair and let Ethel escape.
Combat
Attacks and abilities
Mask of Terror must succeed a DC 17 Wisdom saving throw at the start of each of his turns or become affected by . If he succeeds the throw, he will pass the turn without harming the party.