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Ethereal Guards are the class of enemy that spawns in Mega Fortresses. There are five types of Ethereal Guards, type being unique to whatever Final Son lay at the end of the Mega Fortress. They are pretty tough, and are similar to Dungeon Crawlers and how they spawn in Dungeons. Each Type contains three enemies.
Spawn in The Harbinger's Mega Fortress
Bony: Tough exterior and look like a skull tank. Opens its mouth occasionally and fires a stream of bullets at the closest player. While their mouths are closed, they are Invulnerable. Can only move in straight lines and must turn abruptly to move. Have 500 health.
Carcass: A white, cracked, Basic Tank with a Draconis Rex-like tail that swings off the back, each separate spine made to look like a vertebrae. The number of these tail spines can vary, but no more than ten will appear on a Carcass. If approached from behind, they will whip their tail and do Body Damage. Can use their tail to deflect some bullets. Can take quite the brunt of damage, at 1000 health.
Ghoul: Tiny white Minions that will build up around a player in a ring shape, rotating around them while firing inward on the player. There is usually a large number of these in a room. Have slightly more health than an average Blue Pentagon, at 250.
Spawn in The Warhand's Mega Fortress
Footman: A Lancer tank with a shield-like extension on the back. An Auto Turret sits on top of it. Will chase down and charge at players. Has quite a bit of health, and can quickly turn to absorb damage in its shield, but not all the time for obvious reasons. Has a hefty 700 health.
Field Medic: Shaman-like enemy that constantly heals all enemies in the room for 2% of their Max Health every second. When in danger, can run away, teleport if cornered, and fire slow, but Poisoning shots. Have around 1200 health, slightly more than a third of an Alpha Pentagon.
General: Having a rarer spawn than other enemies in The Warhand's set, Generals have very high health, and wear golden helmets with the same design of The Tres Geminos. They have Destroyer cannons on their front, and spawners on their back that summon Footmen every few seconds. Have 2000 health each, being one of the toughest Ethereal Guards. The Warhand is known for having very tough minions.
Spawn in The Pestilent's Mega Fortress
Badfly: Small red tanks with no barrels and two fly wings. They will fly quickly into players, dealing Body Damage and inflicting Slowed. Usually come in large numbers. When killed, four bullets are fired outward in the diagonal directions. Have 200 health.
Host: Rotten green Overseer with the barrel of a tank that summons a number of Badflys from its death position when killed. Their main attack is actually a laser beam that sweeps 45 degree angle every few seconds. After two sweeps of their laser, they will summon two Badflys from each of their barrels. Will run away if players get too close. Has 700 health.
Plague Doctor: White tanks with plague doctor masks that will run in random cardinal directions (can only move in straight lines), occasionally stopping to fire three green glowing blasts straight forward that do fast and quick damage. Each shot in their three line blast does more damage than the last. Their full attack actually takes about 2 seconds to complete. Have a tough health pool though, at 1100.
Spawn in The Siphon's Mega Fortress
Antimage: Can go invisible for a few seconds every once in a while. They are black tanks with gray gas masks and two Auto Turrets, one on each side. The left one has a lance and the other one a regular barrel. If pricked with the lance, the Antimage will become visible if invisible, the player will be Paralyzed for 2 seconds, and three other random Debuffs will be administered to them that last 6 seconds. They also lose 10% of their Max Health temporarily. They can survive combat quite well, with 550 health.
Gazer: Stationary tank that slowly rotates while firing a random number of evenly spaced lasers from evenly spaced barrels on its body. Can have one, two, three, or four laser barrels on it. Have the highest health of any enemy that can spawn in a Mega Fortress, at 2400. Usually in the center of a room.
Dark Ones: Black Mage-like tanks that fire four blasts in quick succession before teleporting somewhere else in the room. This makes them like Cultist Dungeon Crawlers. The difference is, that with each Dark One killed, the brightness for the entire rest of the Mega Fortress goes down by 2%. Caps at 50% less brightness. Can be particularly annoying, as they have a tough health pool of 750.
Sticking to their themes:
The Harbinger has mainly skeletal enemies, as he is the equivalent to Death. His Fortress is almost entirely made of bones as well.
The Warhand has soldiers and generals. His enemies have the highest health. He is the equivalent of War, after all.
The Pestilent has flies, and hosts of diseases, as well as the doctors who failed to treat the diseases. This makes sense, as he is the equivalent to Pestilence. His Fortress is diseased and full of pools of parasitic waste.
The Siphon's enemies steal your energy, like the Antimage, administering debuffs and draining you of health. Other enemies keep you constantly on the move. The Fortress he resides in is emaciated and deprived of resources. He is the equivalent of Famine after all.