One of the Doomed
They are a type of enemy which is encountered late in the Game Styx: Master of Shadows.
The Doomed appear as flying, spectral elven Heads.
The best course of action when encountering one of the Doomed would possibly be to send a clone in to gather the Spirits attention.
If they catch up to Styx or one of his clones they'll explode and can do a lot of Damage, so it is safer to sacrifice a clone than Styx' own life.
The Doomed are also occasionally referred to as Damned, or "The Damned" in the game at certain points in the game, such as during the loading screen where one of the hints in the lower left corner will read "When The Damned, the spirit of an elf targets you, sacrificing a clone is the most effective means of making an escape."