Hell Guard

Hell Guards are three powerful denizens of Hell encountered in Doom (2016) only at the end of the Necropolis, who must be defeated in order to acquire the Crucible. They are a sort of biological mech, only given life when controlled by a diminutive parasite.

In-Game Description
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''The borderlands between Titan's Realm and the Great Steppe is home to one of the most ferocious demons yet found - the (biomech) Hell Guard. While there has been no direct human contact with the beast it was discovered by Scout-Bots during the second Project Lazarus Tethering Operation (MTC 2147/016). Confronting this demon signalled the end of the expedition, but the Scout-Bots did manage to send data to the relay beacons before they were destroyed.''

''A detailed study of the few video frames that were captured before the equipment was destroyed revealed that the Hell Guard is actually a parasite housed within a bone and keratin exoskeleton. The parasite, imbued with enourmous quantities of Hell energy, provides power to the Hell Guards and controls its actions entirely. Before entering the exoskeleton, the Hell Guard exoskeleton is a lifeless husk. Attempts to retrieve the parasite alone has proved fruitless as the uncoupled demon is too weak to survive passage between dimensions. Once coupled however, the demon presents a daunting and relentless enemy.''

Appearance
The parasite that controls the exoskeleton itself has a fleshy dark red color and is covered with eye-like light green blisters. It has a round mouth with four mandibles around it.

The exoskeleton is a tall, menacing figure which resembles a humanoid creature with no head or neck. It has hooves as feet, like the Baron of Hell and its legs are light brown and flesh red like the rest of the exoskeleton. The chest area has a green radiant rune embedded on it and can open up to give the parasite access to the biomech. Its shoulders possess one shield-like extension covered with spikes on each side to protect the joint that connects the body to its bulky arms which end in claw-like, red hands. Slate-coloured bumps cover the dome on top of the Hell Guard where its head would otherwise be, resembling eyes (though it's unknown if these bumps merely serve as decoration, or work as some kind of biological optics for the parasite inside).

Hell Guards are capable of wielding different weapons, which determine what they can do; a staff, a mace, or even both.

Combat Characteristics
The player will meet three Hell Guards, and will fight with them in two stages:
 * The first Hell Guard is armed with a mace and a staff, as well as a green bubble shield that renders it invincible to weapons fire. This Hell Guard can only be harmed by counter-attacking it; the biomech only drops its shield when it's currently attacking the player, meaning one must dodge the creature's attack and then quickly shoot it until it recovers and pulls its shield back up in order to damage it. This one has a multitude of attacks and maneuvers:
 * The Hell Guard will quickly lunge towards the player and slam its mace on the ground. Occasionally, before doing this, it will move to the opposite side of the arena and slowly strafe around the edge of it before running at the player and performing the attack.
 * The Hell Guard holds up its staff and charges a ball of energy while tracking the player. This ball eventually turns yellow and launches itself at the player in the form of a horizontal spread of three fast-moving fireballs. If the Hell Guard's too close to the player when it chooses this attack, it will perform a somersault away from them before landing and starting the attack.
 * The Hell Guard runs at the player and performs a horizontal swipe with its mace.
 * The Hell Guard moves to the center of the arena and begins rapidly spinning like a dreidel, releasing curved jets of fire that swirl around the arena in a circle. Later in the fight, it may occasionally pause for a moment after the attack is finished, then immediately perform it again with the direction of the jets swapped.
 * The Hell Guard throws its mace like a boomerang towards the player, causing it to fly in a semicircle around where the player was standing when it was thrown and then return to the Hell Guard. The mace itself releases fireballs towards where the player is supposed to be while in flight.
 * After defeating the first Hell Guard, two more will appear; one armed with a mace, the other, a staff. The fight against these two Hell Guards is more intense than the previous due to there being more than one, but this is compensated for by the fact that neither of them have a shield, and can thus be shot at any time. They mostly use the same attacks as the first one (with the one with the mace only using melee attacks and the one with the staff only using ranged ones), though slightly different:
 * The flame-spiral attack performed by the Staff Hell Guard can now be performed anywhere in the arena instead of only at the center, and will slowly spin towards the player instead of spinning in place.
 * The Mace Hell Guard will perform melee swipes more frequently and faster than the first Hell Guard did its.

Tactical Analysis
Defeating the first Hell Guard takes a bit more strategy than Doom's other boss fights; the boss itself is protected by an impenetrable shield at all times that will only temporarily lower when it attacks. Thus, the player's best options are are weapons in their arsenal that can perform a large burst of damage in a single trigger pull, something the Super Shotgun and Gauss Cannon are superb at. This leaves the player with a simple pattern; wait for an attack, dodge said attack, shoot the Hell Guard, and repeat. This should be easy to master, as, at first, the first Guard only performs simple attacks, such as its fireball and melee ones, with the more intricate ones only being utilized when most of its health is gone.

As the fight itself contains no health or armor pickups, the only way the player can restore health (without using Siphon Grenades) is by stunning the Hell Guard, either by shooting it with the BFG, or damaging it when it's preparing its fireball attack, causing the fireball its charging to explode in its face. During this time, the Hell Guard will be immobile, lower its shield, and drop a fair amount of small health pickups every time it's damaged (the Armored Offensive rune does not  appear to also make it drop armor shards).

In the second phase, when the player has to fight two at once, the fight is less like a duel and more of a chaotic game of cat-and-mouse; the player will be perpetually backpedaling from the Hell Guards, as at least one of them (if not both) will be constantly chasing them almost the whole fight. The player can rely on constant-damage weapons like the Chaingun and Plasma Rifle at this point, as neither Guard has a shield and thus are always vulnerable to damage. Both Guards can still also be stunned, either one at a time to allow focusing on the other for a moment, or both at once to give breathing room.

However, an alternate strategy is using any gun upgraded with piercing shots and firing through the wand Guard towards the mace Guard, as it will damage both targets at the same time. The fully-upgraded Super Shotgun will end the fight in less than a minute.

Trivia

 * The second phase of the Hell Guard fight (where the player fights two Hell Guards at once) is likely a nod to E1M8: Phobos Anomaly of the first Doom; the game's first boss fight is against a pair of Barons of Hell, often called the "bruiser brothers."
 * The mace weapon used by two of the three Hell Guards resembles the Guardian of Hell's own maces from Doom 3.
 * Each Hell Guard has their own unique glory kill animation:
 * Against the first Hell Guard, the Doom Marine forcibly yanks its staff out of its hand before jamming it into the cockpit like a spear, shattering the front of the carapace and likely crushing the parasite inside.
 * The second Hell Guard has the Doom Marine climbing on top of its right shoulder and punching the top of the cockpit several times; eventually, his fist punches right through it to crush the parasite inside.
 * To finish off the last Hell Guard, the Doom Marine physically rips open the cockpit, revealing the parasite inside. It briefly growls at the Doom Marine before he forcibly wraps his arm around it, tears it out of the Hell Guard, and proceeds to pull on its head and tail until he rips it in half.