Shadowfall

Shadowfall is a rare creature located in Algoroc. It drops Dusk Guard. At the moment however it appears he can glitch and be unable to be fought. If he's glitched he will be higher in the sky if you're under him look up and you may see him. It is possible to aggro him even when bugged by having a scientist scan the loftite crystals in Tremor Ridge and using the jump buff to reach him.

Location
Shadowfall can be found flying above a ridge between Tremor Ridge and Gallow around (3580, -4252). He can get close enough to the ground to kill near Tremor Ridge at (3835, -4286)

Achievement
Killing Shadowfall is part of the I Like it Rare: Algoroc achievement.