Spirit

The Spirit is an enemy introduced in The Ancient Gods - Part One, the first downloadable content expansion for Doom Eternal. It is a blue, ghostly version of the Summoner from Doom (2016) that possesses other demons to empower them. Once the possessed demon is slain, the Spirit bursts out and becomes vulnerable for a short time; however, only one specific weapon can harm it.

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
A Spirit will already be possessing a demon when it spawns; its host can be identified by its blue-glowing eyes. The player must kill the possessed demon to release the Spirit, and then eliminate it with the Plasma Rifle's Microwave Beam mod, expending 75 cell ammo. A released Spirit will immediately look for another non-Ambient demon (usually the strongest one present) and begin possessing it, which takes a couple seconds of uninterrupted channeling. If successful, the Spirit will possess the targeted demon, forcing the player to kill it before having another chance to destroy its possessor.

A demon possessed by a Spirit becomes faster, more aggressive, and nearly immune to plasma-based weapon damage like the Plasma Rifle and Ballista. If it has weak points, they are rendered unbreakable; it also cannot be faltered, frozen, or killed by the Chainsaw. Contrary to popular belief, possessed demons do not gain buffs to their health and attack damage; instead, the damage they receive is reduced by a certain percentage (e.g. a Hell Knight will take half the damage from rockets, thus taking six rockets to kill instead of three) and they move and attack much faster. The scope of these bonuses varies between demon types, but they always make a possessed demon an extreme threat that will not stop attacking at full power until the player outputs enough raw damage to kill it outright.

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 helpless. However, this method uses cell ammo, so care must be taken to have 75 or more before the Spirit is released. Additionally, Spirits have a very specific window of vulnerability due to the way their spawns are coded; when a possessed demon is set to spawn, the game spawns it first and then possesses it with a Spirit. While difficult to exploit, it is actually possible to freeze or handicap the host during this time, giving the player an advantage.

If destroying a Spirit is infeasible, the player's only alternative is to kill every other demon in the area. Spirits cannot attack the player directly; if there are no more demons to possess within their range, they will simply vanish.

Spirits also play a role in the battle against the Seraphim in The Holt. During the second and final phases of the battle, Samur will summon two Spirits and use their energy to shield himself. The player must kill the demons the Spirits possess (a Hell Knight and a Mancubus the first time, a Dread Knight and a Pain Elemental the second time) and then destroy the Spirits themselves to lower Samur's shield and leave him vulnerable.

Trivia

 * Hugo Martin compares the use of the Microwave Beam against the Spirit to the proton pack of the Ghostbusters series.