When I was finished with the sprites, I stumbled upon some other mods that use the spirit sprites, so I tried to make my attacks somewhat unique. The first idea was to make it charge at the target like the Lost Soul, but for a short distance. When I later added ghost sounds to the monster, I coincidentally added a charge sound with a duration exactly as long as the charge attack. So I left it as it is. But the sound was always cut off when it charged from a short distance. To prevent that, I made it shoot projectiles at close range. Projectiles at close range? Yeah, I was lazy again and didn't bother to furthermore edit the sprites in order to make it attack with claws or teeth. Although some may people may disagree, I think this "workaround" works pretty well.
The Lost Spirit may not fit into Doom, but I hope somebody can find use for it in a "fantasy" mod or something like that.
Name: Lost Spirit
Distance: Projectile, Charge
Brighmaps: No (full brightness)
Sounds: Wolfsinger, CosmicD, Robinhood76
Sprites: Raven Software, id Software
Sprite Edit: DeVloek
Idea Base: Lost Soul
Similar to the Lost Soul, the Lost Spirit will charge at its target, but the duration of the attack is limited to about 1.5 seconds. When closer to the target, it shoots two projectiles with a limited range. The Lost Spirit deals more damage and has more health than the Lost Soul. It is a ghost, so it feels no pain and it can fly through anything (only collides with other actors during the charge attack).
Because the sprites are based on a monster that has only two frames (of which a few were just mirrored), I edited the sprites to make the animation look less wonky, and added more frames so it doesn't look too repetitive.
The sounds are completely new and unique, taken from http://www.freesound.org.