Diablo
Diablo, inspired by roguelikes and Dungeons & Dragons, was originally designed as a single-player, turn-based game by Condor Entertainment (later, Blizzard North). When Allen Adham and I heard about the game we thought it would be awesome if it were real-time and multiplayer. We inked a deal with Condor and immediately set about convincing them to change the game.
At the time the game was being developed, game magazines (made of paper -- this was the old days) were pushing the idea that "the role playing genre is dead". When the game shipped in December 1996 it became a global phenomenon. Initially derided as an "action RPG", which is to say, not a "true RPG", it has since become the definitive template for the RPG genre.
When developed wasn't progressing, I flew to Blizzard North to implement multiplayer for them. It was ... an ordeal. 14+ hour days, 6-7 days per week for four months. Would not recommend. In addition to writing the multiplayer syncing code and integrating with battle.net (conceived and written by Mike O'Brien), I also wrote the sound player, some of UI code, (rewrote) event-handling, and -- as I later did with StarCraft fixed an enormous amount of bugs, oh yes I did.