Bro Hammer
From Halopedia, the Halo wiki
Brohammer
Brohammer is an achievement in Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection. It is awarded for completing any level cooperatively on Legendary difficulty. It is worth 50 gamerpoints. Receiving this achievement also unlocks the "Standard Operating Brocedure" and "Brovershield" achievements if the player hasn't completed a level cooperatively on Normal or Heroic, respectively.
It is represented by two Sangheili skulls over the Heroic difficulty shield.
Trivia
The achievement is a reference to Carol Rawley's callsign, "Foehammer".
