|Location:||Black Powder Mine|
|An unfathomable horror from the abyss, feared by even the strongest Gundead.
Killithid have the ability to dominate the guns of others, turning them against their wielders. Only the most iron-willed gunslingers can hope to escape being enslaved by their power.
Mine Flayer primarily stays near the center of the arena during the fight, and has several attacks:
- Fires large rings of bullets that expand and drift to the sides.
- Fires two rotating streams of bullets in all directions.
- Spins around, throwing claymore mines around the arena that will either chase the player or walk in circles around the center of the room. They explode upon contact and cannot be shot directly, and will disappear after a while. During this attack, the Mine Flayer fires spreads of bullets at the player.
- Fires bouncing bullets in all directions.
- Disappears, leaving behind many bells. Killing the correct bell causes the Mine Flayer to re-appear, while killing an incorrect bell causes it to explode into a ring of bullets.
- Mine attack: Move in circles through the periphery of the room, and don't stop moving. If it is necessary to cross paths with a mine, roll towards it right as you enter the mine's perimeter to avoid damage.
- Bell attack: It's impossible to tell which bell the mine flayer hides under. Take them out one at a time, or with explosives.
- Bouncing bullets: Simply move back and forth to dodge the bullets, and avoid dodge rolling.
- Rotating streams of bullets: Stay near the walls of the arena where the gaps in the bullets are largest. Avoid dodge rolling.
- Drifting rings of bullets: Stay near the bottom of the room, as the rings tend to drift to sides. This way, minimal movement and dodge rolling is required and you can use this attack to freely attack the boss without interruption.
- Most of the bullet attacks can be easily dodged if there are pillars in the room. Keep in mind that when the attack being deployed is rotating bullets, the player must stay strafed to one of the sides of the pillars, since the bullets can curve and reach behind the pillar itself.
- It is recommended to drop Devolver Rounds in case the player has them. It affects Mine Flayer's bells and will devolve them into Arrowkin who will remain even after the right bell has been shot.
- If the player's curse is high enough, the mines or the bells spawned by the Mine Flayer may be Jammed, increasing their movement speed and dealing 1 full heart of damage.
- The mines and bells can be inflicted with debuffs, meaning they can die by the Poison and the Burn debuffs.
- The Mine Flayer himself, however, has a sponge-like effect, so he cleans up any goop around him, making him functionally immune to any fire or poison goop. Keep that in mind if you have the Molotov or the Poison Vial. He is actually not immune to the effects themselves, however, so he can be lit on fire, frozen (as far as bosses can, anyway), or poisoned by effects such as Irradiated Lead or Hot Lead, or direct hits from weapons like Flame Hand or Poxcannon.
- Before they land, the mines are considered projectiles, meaning they can be destroyed by anything that can remove projectiles, such as blanks, the Composite Gun or the Dark Marker's charged attacks, etc.
- IBomb Companion App will destroy all mines and bells.
- The BSG will also destroy the mines.
- The Mine Flayer is a boss that was added in the Supply Drop Update.
- He is the only non-secret, non-past boss to be added in an update, rather than being in the original release.
- The Mine Flayer's name either alludes to his location (in the Black Powder Mine) or the fact that he throws claymore mines during his boss fight.
- The Mine Flayer's bell and appearance are references to a race of creatures from Dungeons and Dragons, called the Illithids, or more commonly, Mind Flayers.
- The Mine Flayer's appearance, mainly the head, is also a reference to Cthulhu, a fictional Lovecraftian horror monster, described by Lovecraft as "A monster of vaguely anthropoid outline, but with an octopus-like head whose face was a mass of feelers, a scaly, rubbery-looking body, prodigious claws on hind and fore feet, and long, narrow wings behind."
- The phrase "Sinister Bell" is also a reference to an item from Bloodborne.
- The Mine Flayer may be a reference to the Mind Flayer enemies from Demon's Souls, who also carried bells.
- The Mine Flayer generates his attacks by shooting the bell he holds with a handgun, producing various ammomantic resonances.