A death mask is an image, typically in wax or plaster cast, made of a person's face following death, often by taking a cast or impression directly from the corpse. Death masks may be mementos of the dead, or be used for creation of portraits.
The appearance of the Death Mask in Portrait of Ruin seems to be more akin to the heraldic depiction of cherubim, albeit with an extra pair of wings and being an adult.
The Death Mask is a demon that appears as a feathered human face covered in blue feathers, which compose its "hair" and "beard", that flies around while unleashing ice projectiles upon the heroes. It possesses four wings that protrude from each corner of its body and is only found in the City of Haze.
It suddenly materializes on the screen and starts flying about in an attempt to inflict contact damage. Soon enough, however, it will stop and start channeling its dark powers to unleash a barrage of ice shards in the player's general direction.
Due to its demonic nature, it is resilient to Dark attacks, so spells like Dark Rift will do decreased damage. However, a spell such as Piercing Beam will do critical damage, due to its Holy element being the antithesis to the demon's dark element.