The Brug Mass is a swarm of countless Brugs gathered around an Emperor Brug, forming a vaguely humanoid battle body. It is the first boss in Metroid: Other M, fought in the BOTTLE SHIP Control Room.
Shortly after Samus Aran and the 07th Platoon reach the Control Room, they encounter the corpse of a Lab Worker. A Brug crawls out from under it - Lyle Smithsonian panics, kicks the creature away, and shoots at it until he is stopped by James Pierce. As Samus starts to talk to Adam Malkovich about the Platoon's mission, hundreds of Brugs crawl out of the walls and ceiling, gather around the eye-like Emperor Brug, and coalesce into the Brug Mass.
The Brug Mass uses its two tentacles to attack, rearing up and roaring before striking at Samus. It will occasionally perform a spinning attack when the soldiers get too close. Hitting the Brug Mass with a Charge Shot reveals that she and the soldiers cannot harm it, forcing Samus to SenseMove in Search View to avoid a blow. Adam then authorizes the use of both the soldiers' Freeze Guns and Samus' Missiles.
Samus must then hit the Brug Mass in the eye with a Charge Beam, Missile, or Overblast to goad it into performing an extra heavy blow. Its tentacle will get stuck in the floor, allowing the soldiers to freeze it and leave it susceptible to Samus' Missiles - if Samus does not quickly destroy the frozen limb, it will thaw out. Additionally, if Samus stands too close to the foot of the Brug Mass, she will be immobilized by Brugs before being tossed into the air. If she fires a Charge Beam midair she can Counter Attack and escape, but if she fails then she will be pierced by its tentacle and thrown down for heavy damage.
Once both of its tentacles have been destroyed by Samus' Missiles, the Brug Mass will attack with its torso by jumping or body slamming. After a body slam, the soldiers will then freeze its foot - once Samus destroys the foot, the Brug Mass falls apart to reveal the Emperor Brug. The Emperor Brug scuttles around and can be easily finished off by normal Power Beam shots to end the battle. If Samus fails to do so in time, it will dive back into the Brugs and the Brug Mass will reform, albeit lacking both arms.
- The Brug Mass was first shown in the original Metroid: Other M trailer at E3 2009.
- The concept of the Brug Mass's eye being a larger, controlling insect is reminiscent of the Armogohma from The Legend of Zelda: Twilight Princess.
- The Brug Mass was referred to as Unidentified Life Form 27 in the UK's Official Nintendo Magazine: "An early encounter with a giant one-eyed monster (Unidentified Life Form 27)". "ULF 27" was the Brug Mass's unofficial designation on Wikitroid prior to the release of Metroid: Other M Premiere Edition, under its naming policy at the time for unnamed creatures and subjects in the Metroid series. This official publication was one of the factors that ultimately led to Wikitroid repealing the naming policy.
- If Samus waits long enough after the Brug Mass loses its first arm, one of the soldiers may attempt and fail an Overblast. It is impossible to tell which soldier does it, as they all have identical models during battle (with the exception of Anthony). This is an intended hint for the player to make Samus attempt an Overblast herself.
- At the start of the Brug Mass battle, it uses the same theme as Fune and Namihe, as well as Nightmare's second battle. Once Adam authorizes Missiles, it switches to the Brug Mass's unique battle theme for the remainder of the fight.