There’s a particular location in Halo: Reach where you can find a partying Brute who wants to have a good time. Unlike the rest of the Covenant attacking Reach, he wants to play some sick music, and not lay down fire.
You can find the DJ brute during the New Alexandria level in Halo: Reach.
You want to proceed through the mission as you normally would until you receive instructions to make your way to Club Errera. It won’t tell you to go to the club, but it’ll be the building with red lights around it after you defend a group of marines against some Hunters.
Turn around and make your way back to the hospital, the building you cleared out and took out the first jammer. Lower your Falcon down to altitude 360 to 380, and near the bottom, you should see a small platform with two trees on either side, standing against two white pillars, pictured below. Jump out of your Falcon to go to the green button, pressing it. When you approach, it should tell you “Press to Open Door,” and nothing happens.
Now, proceed back to the Club. There are two turrets you need to take out before landing. After eliminating them, head inside and you should hear club music playing. When you proceed further in, you’ll notice grunts are harmlessly dancing around with Brute located close to the entrance working a DJ table. Stand next to him for a few seconds, and you unlock the HBOrg achievement for Halo: Reach.