Section III ARG
From Halopedia, the Halo wiki
The Section 3 ARG was an alternate reality game done by 343 Industries. The ARG was a campaign that spread out through Halo 4 and Halo: Spartan Assault. It was a forum-based campaign that also spread into chat groups, trailers, and even in Halo 4.[1]
Summary
The campaign lasted for two years, giving fans a look into the Office of Naval Intelligence's Section Three and also insight into the experiments they did. Users found glyphs that led to codes being discovered for use on Halo Waypoint (now only on the Xbox 360). While the users found codes, an AI that was originally meant to help them turned against them, giving the puzzles for the ARG.
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