Spirit

The spirit is a blue, ghostly version of the summoner from Doom (2016) which is slated to appear in The Ancient Gods, Part One, the first expansion for Doom Eternal. It possesses other demons in order to specially empower them, and has been seen in the trailer bursting out of slain demons that give off a distinctive blue glow while alive.

Tactical analysis
In the trailer, the spirit is seen emerging from a slain tyrant which had been giving off a blue glow to possess a soldier, which immediately starts glowing in the same manner. According to Hugo Martin, the possessed demon becomes nearly immune to ballista damage, 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. Possessed monsters have more hit points and attack much faster, making them an extreme threat. 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

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