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 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 begins shooting at it before he is instructed to stop by his comrade James Pierce. As Samus starts to talk to Adam Malkovich about her assistance with 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. If Samus goes too close to the Mass, her foot will get caught in it and the Mass will throw her up and use its tentacle to strike her down, dealing heavy damage.
Initially, the Marines 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 Marines' Freeze Guns and Samus' Missiles. Samus must fire a Charge Beam shot, Missile or perform an Overblast at the Emperor Brug, allowing the Marines to freeze the tentacles of the beast. Once a tentacle is frozen, Samus can fire a Missile at it to destroy it. With one tentacle, it attacks by with its repetitive strikes, drags its arm across the floor, and striking down the floor while spurting the tentacle through the floor at a long distance. After both tentacles have been destroyed, the base of the creature becomes vulnerable to attack and can be destroyed in the same fashion as the tentacles. At this point, the creature will begin slamming its whole body into the ground in an attempt to injure Samus or one of the Marines. It can also spin its body very quickly and jump. After the base has been destroyed, the Emperor Brug falls out of the Mass and Samus must kill the eyeball to finish off the beast. Once the Emperor Brug is killed, the remaining Brugs in the mass scurry away.
If Samus stalls to dislodge the frozen part of the Mass, it will unfreeze and the Emperor Brug must be shot again. If she takes too long to kill the Emperor Brug, it will reattach itself to the Mass and the Mass' base must be broken off again. Also, the Emperor Brug, after being dislodged again, will have its health completely restored.
- The concept of the Brug Mass' 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 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)".  This was its designation here on Wikitroid, under its naming policy at the time for currently unnamed creatures and subjects in the series.
- If Samus waits long enough to attack the Brug Mass after one arm is dislodged, one of the Marines will attempt an Overblast, but the Mass will simply shake off the Marine. The same Marine will keep trying exactly the same method over the course of the phase but will always fall short.