Skullwing

The Skullwing is a type of demon.

In-game
The monster is found mainly in Diablo: Hellfire. It looks like a demonic skeleton with wings, and has magic attacks, while dying with a large flare. Despite its official implementation in Hellfire, its graphics and even most of the monster implementation already exist in the original Diablo executable and data file, but can only be enabled within the game by modifying the Diablo.exe file.

Hellfire Version
The Hellfire variant is a relatively powerful being that resides in the Crypt Level 1-2. Along with its claw attacks, it fires magic missiles that do significant damage. The attack type of this monster is the same as Magma Demons.

The monster is an undead, but a special case: it is resistant to Holy Bolt. In addition, it has resistances to both elemental types of spells: fire and lightning, so the only recommended spells against it are Stone Curse, Apocalypse, Golem, Berserk, Flash, Blood Star and Bone Spirit.

The Hellfire skullwing is a basic version of the Bone Demon. The skullwing doesn't use the magic attack or Holy Bolt resistance of its predecessor however.