Hell guard

The Hell guard is a biomechanical golem-like monster in the Doom inhabited by a small, worm-like demon which controls the larger body. Three of these entities serve as the guardians of the crucible at its resting place deep inside the Titan's Realm, and act as the boss of that level.

Tactical analysis
Hell guards are permanently protected by shields that only dissipate when they attempt to attack. They can be staggered by detonating the charging yellow orb right before multiple orbs are fired toward the player in a cone. They are weak versus explosive ammunition.

Lore
The UAC was never able to capture a Hell guard due to their power and ferocity, nor were they able to isolate the parasite that powers the biomech unit, as it was too fragile to survive transportation on its own. The first Hell guards were encountered by UAC scout-bots during the second Lazarus tethering operation with designation 2147/016, between the Great Steppes and the Titan's Realm. Only minimal data was collected before the bots were destroyed.

Pre-release
Pre-release versions of the game contained strings which referenced two monsters referred to as "Olivia's Guards," which were to be named and Apollyon. Other strings in the executable indicate that they were to cooperate in the same way as the Hell guards do when there are two present, and that they had the same attacks and actions, including a "tantrum" wherein they strike the ground with their club, a "pool ball" attack which ricochets them around the arena while protected by their shields, and beam and hammer attacks. From this, it is evident that Olivia's guards were repurposed into the Hell guards for the final release version.

Information in The Art of DOOM indicates that the earliest concept for the Hell guards was originally called the "Twins", and was meant to be two demons inhabiting a single body and competing violently for its control. When challenged in battle, the two would sometimes split apart temporarily by projecting a separate mirrored form for the other demon to control. Concept art for a worm-like demon called the "Abaddon pilot," significantly larger and more developed than the final game's parasite, is also depicted in the book.