In the week or so after the release of The Binding of Isaac: Afterbirth [official site], many members of the Isaac community were up in arms. Datamining - digging through the code for behind the scenes information - had revealed less new content than the pre-release build-up and Steam store page seemed to promise. There was talk of gated content that would only release when updates arrived, there were accusations of lying and betrayal; the Isaac subreddit was saltier than the Dead Sea. And then, over the weekend, the game crept out into the real world and everything got a little weird...
The community had suspected an alternate reality game (ARG) might be happening for a while. The discovery that donation boxes in the new Greed mode of the game would only accept a total of 109 coins was the most obvious clue. 109 is an important number in the weird world of intense Isaac fandom, having been the number of hours taken by dataminers to discover hidden character The Lost in its remake Rebirth. There were other instances of '109', including an official stream of the game crashing at 10:09 and a patch arriving 109 hours after release.
But all efforts to discover more came up short. Until, that is, one of the achievement images was updated. Once the image had been cracked (by MetalAxel, who explains all over on reddit), it led to an imgur link containing a picture of Isaac and the quote: "And he removed that day the he goats". That's from Genesis, one of the books in the big ol' book that is Isaac's origin story.
That's when things started to move quickly. Isaac co-creator Edmund McMillen cryptically pointed those playing along to eighties vampire movie The Lost Boys, which eventually led a couple of intrepid investigators to a specific location in Santa Cruz. One of them found this. From there, voice mail messages (with backward sections, obv), and a series of mysterious questions and clues eventually led to this little fella, buried near a building in Santa Ana. Street number 109, of course.
(source: 'mikeyninja', reddit)
The text on Greed's head is the ascii code for an '@' symbol, which pointed folks toward Twitter. The account name was written on the buried Isaac's body. The community gave Isaac's body a voice, by tweeting a series of questions, and were rewarded by an update to the game that unlocked a previously unmentioned character and associated items.
McMillen says he'll write a postmortem of the whole experience soon. For now, he's keen to make clear that the events of the ARG were in some way canonical. Splendid stuff.