Almost twenty years ago, a game developer just getting its first real taste of success released an innovative and darkly beautiful game with a simple premise: click on monsters until they die. Collect the weapons and money they drop. Use this to improve yourself. Then, go on to kill more monsters until you get to the strongest one of all: the devil himself. Wash, rinse, repeat.

The game was called Diablo, and with it, Blizzard Entertainment helped change modern videogames. But when the game came out in 1996, the company was...