The Brug Mass is a vaguely humanoid collection of many Brugs gathered around an Emperor Brug (which possesses a large eye on its body). It is the first boss in Metroid: Other M and is fought in the BOTTLE SHIP Control Room.
Shortly after Samus Aran and the 07th Platoon arrive in the Control Room, they encounter the corpse of a Lab Worker whose lab coat is covered in green blood. A Brug soon crawls out from under it - Lyle Smithsonian panics, kicks the creature away, and shoots at it until he is instructed to stop by his comrade 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 Emperor Brug, and coalesce into the Brug Mass.
The Brug Mass uses its two long tentacles to attack and strike down Samus. It will occasionally perform a spinning attack, usually when the soldiers get too close. Initially, the soldiers and Samus are unable to damage the Brug Mass, and only come to realize that if Samus SenseMoves in Search View. Adam then authorizes the use of both the soldiers' Freeze Guns and Samus' Missiles.
Samus must then fire Charge Beam shots or Missiles, or perform an Overblast on the Emperor Brug, in order to goad it into performing an extra heavy blow. Its tentacle will then 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 eventually thaw out. During this phase the Brug Mass continues to strike with its tentacles normally. Additionally, if Samus stands too close to it for too long, 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 onto the floor for heavy damage.
Once both of its tentacles have been destroyed by Samus' Missiles, the Brug Mass will begin to attack with its torso by jumping or body slamming the floor. After a body slam, the soldiers will then freeze its base - once Samus hits the base with a Missile, the Brug Mass will fall apart, separating the Emperor Brug and the Brugs. The Emperor Brug scuttles around and can be easily finished off by normal Power Beam shots. However, if Samus stalls to do so, it will eventually dive back into the Brugs and the Brug Mass will reform, albeit lacking both arms and with low health.
- The concept of the Brug Mass's eye being a larger, controlling insect is identical to that of the Armogohma from The Legend of Zelda: Twilight Princess.
- Until Adam authorizes the use of the Freeze Guns and the Missiles, Samus is unable to damage the Emperor Brug with the Charge Beam as well as jumping on the mass to perform Overblast.
- The Brug Mass was first shown in the original trailer at E3 2009.
- The Brug Mass was referred to as Unidentified Life Form 27 in 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 to attack the Brug Mass after one arm is dislodged, one of the soldiers will attempt an Overblast, but the Mass will simply shake off the soldier. The same soldier will keep trying exactly the same method over the course of the phase but will always fall short. This is likely an intended hint for the player to make Samus try.
- 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.