Halo: Reach Challenge Initiative - Participation Counts
Complete 4 games in multiplayer Matchmaking.
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Though it may seem like a real challenge, just sit through it, controller firm in your hand, 4 games may seem like a high number, but I believe in you, you can do this.
Whatever you do, DON'T AFK Bungie/343 WILL ban you...
Play Grifball. The matches can be Very short so you can get it done in no time. =)