Spirit

The spirit is the essence of a Summoner from Doom (2016) that was once slain by the Doomslayer. This enemy appears in The Ancient Gods, Part One, the first expansion for Doom Eternal. It possesses other demons in order to specially empower them, and causes the demon it possesses to glow blue.

Tactical analysis
The Spirit possesses the body of any demon on the field, granting them enhanced speed, health, and aggression. Its weak points (if any exist) are rendered unbreakable, and it cannot be faltered; the combined effect makes a possessed demon much more resilient than usual and ensures that it will not stop attacking at full power until it is dead. The player must kill the possessed demon first to release the spirit, and then eliminate it with the plasma rifle's microwave beam mod while it is stunned from the host's death. If it is left alone at all, it will immediately look for another demon to possess and the process must be repeated.

The spirit cannot, however, attack the player directly and if there are no more demons to possess within its range, it will vanish.

Lore
Spirits are the deathly essence of slain Summoner demons, a rare caste within Hell. After falling by the Slayer's hand their hatred for him fuels their ghostly state, giving shape to their discorporate essence. Unable to affect the physical world in their new forms, Spirits exist as soul-devouring parasites and rely on others to wreak their violence. Flowing into the bodies of other demons, a Spirit fuses its own essence with that of its host, gifting it with strength and speed. Even after ejecting from a slain host, Spirits cannot be harmed by conventional firearms; only sustained and concentrated energy can affect their essence and grant them a second death.

Trivia

 * The Spirit is the only Demon who must be eliminated with a specific weapon.
 * Hugo Martin compares the use of the microwave beam mod against the spirit to the proton pack of the  series of media.