2002 was the first story, and a new story in the game got released every so often afterwards to make a bunch of different mini-stories to explain the bigger one, which is what's generally referred to as the game. it's a doujinshi game that broke 100,000 sales, and the 2nd to reach that goal. from there, pretty much grew into a bunch of other mediums and at this point the guys who made it are probably too big to really stay in that circle. though for horror visual novels, it's pretty much the go-to if you don't want to sleep. if you want tentacle monsters or demons and the like, you've got plenty of other choices. just none that are really as big.
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The future must protect the clone's past.
"Maybe it's a tentacle, molesting the planet itself. - Aschen Brodel.