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.

Tactical analysis
The possessed demon becomes nearly immune to Plasma based-weapon damage like Plasma Rifle and Ballista, its weak points (if any exist) are rendered unbreakable, they cannot be faltered and frozen using the Equipment Launcher and they also cannot be chain-sawed even if they fall in the chain-sawing category; 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.

Possessed monsters have much more hit points (for instance a Hell Knight will take 6 rockets to die instead of the usual 3) and move and attack much faster, making them an extreme threat. Contrary to popular belief, possessed demons do NOT inflict extra damage. 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 microwave beam mod is also the only means to stun-lock the possessed demon to give the player some breathing time in case of being surrounded by a large crowd of enemies.

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.