Spirit

The spirit is an enemy which first 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 movement speed, projectile speed, damage reduction, and aggression. Additionally, a possessed demon's 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; any weapon or mod other than the microwave beam will harmlessly pass through it. 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 essence of slain Summoners. After falling by the Doom Slayer's hand their hatred for him fuels their ghostly state, manifesting their ghostly aura. Unable to affect the physical world in their new forms, Spirits possess the bodies of other demons and rely on them to wreak their violence. 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 second monster in the Doom series that can only be harmed with a specific weapon, after the Doom 3 cyberdemon which could only be harmed with the Soul Cube.
 * Hugo Martin compares the use of the microwave beam mod against the spirit to the proton pack of the  series of media.