Spirit

The Spirit is a blue, ghostly version of the Summoner that first appears in The Ancient Gods - Part One, the first downloadable content expansion for Doom Eternal. It possesses other demons in order to specially empower them. Once the possessed demon is slain, the Spirit bursts out and gives off a distinctive blue glow while alive.

Codex Entry
''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.''

Tactical analysis
Spirits spawn along with another demon that it possesses. The player must kill the possessed demon first to release the Spirit, and then eliminate it with the Plasma Rifle's microwave beam mod, expending 75 cell ammo. Once released it will immediately look for another non-ambient demon and begin possessing it, which takes a couple seconds of uninterrupted channeling.

The possessed demon gains increased speed and aggression, becomes nearly immune to Plasma based-weapon damage like the Plasma Rifle and Ballista, its weak points (if any exist) are rendered unbreakable, they cannot be faltered or frozen, and they cannot be chain-sawed even if it would otherwise be possible to do so. The combined effects make a possessed demon much more resilient than usual and ensures that it will not stop attacking at full power until it is dead.

The Plasma Rifle's Microwave Beam mod is able to stun possessed demons. By quickly switching between stunning the demon and dealing damage, a possessed demon can be rendered unable to retaliate. However, this method uses cell ammo, so care must be taken to have 75 or more before the Spirit is released.

Contrary to popular belief, possessed monsters don't have more health nor more attack damage, instead, the damage they receive gets reduced by a certain percentage (for instance a Hell Knight will take half the damage from rockets, thus it takes 6 rockets to kill it.) and they move and attack much faster (The attack and movement bonus is not fixed, and each demon has different buffs), making them an extreme threat. An important quirk to remember when dealing with pre-spirited demons is that there is actually a very brief window between when the demon spawns, and when the demon becomes possessed, during which it is actually possible to freeze or handicap them.

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

Trivia

 * Hugo Martin compares the use of the microwave beam mod against the spirit to the proton pack of the Ghostbusters series.