Section 3 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.
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.
The campaign ended on November 11, 2014 after Sec3OP36516, the main handler at the time, stopped posting after having a mini-forum game with the players.
The team was declared MIA after fighting to repel Covenant forces, though before that they sent the intel as a data node to a remote archival station.
Year 1 Glyph Hunting
In 2557, several Forerunner glyph-encrypted data traffic started to happen. The data was detected by Section Three personnel that formed codes. Section 3 also noted to its operatives about other terminals that suggested access to new domain data archives, though the data decryption could only be completed aboard the UNSC Infinity.
Year 1 Time capsule
A Section 3 operative arrives called Sec3OP36516, he scans the Waypoint forums for tags and find them. He then tells the players about valuable Intel that was sent from Reach to a remote archival station, prior to the fall that a spartan team sent, and sends a coded message with a data node (Time capsule). He also suggests players to keep an eye out for transmissions in numerous places.
Starting to use the key to the Time capsule lead to a transmissions detailing it, saying how it was lost for almost 4 years, and that Section 3 operatives are now trying to access it. This lead to the players finding emblems.
SEC3OP1024350 another Section 3 operative posted giving information, and urged the players to ignore the emblems they found stating they were fake and that the timeline did not match. During this Sec3OP36516 gave the players ONI journal extracts that gave them hints on what to do in the puzzles. One hint specifically went on how to avoid distractions which was relating to how SEC3OP1024350 was trying to divert the players attentions.
Sec3OP36516 then went over the events that spread over the past month. Giving the goal to the players that the mission was uncovering data from transmissions to obtain technical specs for both spartan armor and spartan insignia's. Following this several sent PM's about the glyphs on Requiem.
SEC3OP1024350 admitted its account was compromised and with the help of Section 1 it got control again. Sec3OP36516 then said to ignore him, as his account had previously been compromised. Though SEC3OP1024350 announced that the Spartan team who sent that transmitted the data had an infiltrator, he also sends a call-sign. This call sign turns out to be valid.
Players then created a chat page that is then used by the operatives.
Year 1 enter hydrAi
An AI called hydrAi appeared asking for a number, giving curious responses every time one was given. During this time it was stated that Sec3OP36516's account was hijacked by a virus in hydrAi, but after the correct number was given to hydrAi, it freed the virus from it and also allowed Sec3OP36516 access to his account again.
After this, operatives were then tasked by hydrAi to complete 1,000,000 War Games simulations, the reason for this steep number was for required field and simulation data for upcoming decryption algorithms.
Year 2 (2013)
During the obtaining and decoding of the data, HydrAi and other character fought to hide or reveal the details.
Year 3 (2014)
At the end, Section 3 Operatives were successful and the data was recovered, enhancing the armor. This data also helped enhance War Games Simulations.
Items of note
Unofficially some of the handlers came back after they left 343i, this led to some of the players opening some of the cookies that were missed during the original run of the ARG.
Link to a lot of available sources