A brief summary of the ARG is that Jadusable, a college student on 4chan, started playing a Majora's Mask cartridge that was haunted by a ghost called BEN. He posted these hauntings online on youtube and talked about them on 4chan. These hauntings led to a deeper, more sinister origin online with the discovery of the Moon Children, a cult based around worshiping the Moon. Players of the ARG had to decipher codes, solve riddles, and work together to save both Jadusable, and Ryukaki (another guy who had similar experiences).
The first two Arcs of the ARG had already been completed, and Jadusable was about to move on to the third. This is where Within Hubris came in. Jadusable created Oldbris to be an official forum for players to work together and discuss theories. This never got to be used correctly, since Jad never completed the third Arc of the ARG, which was to utilize an actual interactive video game. The forum remained, and this is where we are today.
For a more in depth look at the ARG, check out the Jadusable Wiki.