Short answers, pulled from the story.
As of the 8th of April 2020, the Diablo series had sold nearly 100 million copies worldwide. Diablo III alone sold 3.5 million copies on its first day and 6.3 million in its first week, making it the fastest-selling PC game of all time at release.
David Brevik conceived Diablo around 1994 while working at Condor Games, drawing inspiration from roguelike games. Blizzard Entertainment acquired Condor and renamed it Blizzard North, which developed the game and released it in January 1997.
The Diablo series is set in Sanctuary, a dark fantasy world created by rebel angels and demons. Sanctuary is divided into three realms: the High Heavens, the Burning Hells, and the human world. Recurring locations include the town of Tristram and the region around Mount Arreat.
Blizzard closed Blizzard North in August 2005 following a dispute with parent company Vivendi Games that had driven key figures, including Diablo creator David Brevik and founders Max and Erich Schaefer, out of the studio by 2003. Most of the existing work on Diablo III was scrapped when the studio closed.
Two notable projects were cancelled. Diablo Junior, a handheld prequel planned for Game Boy, was scrapped due to high production costs after being moved through multiple target platforms. Project Hades, a Dark Souls-style third-person Diablo game led by Josh Mosqueira, ran from 2014 to 2016 before being cancelled when Mosqueira left Blizzard.
Time magazine named Diablo the second most influential video game character of all time. Guinness World Records ranked him 44th on its 2013 list of top 50 video game villains, and GamePro placed him 14th on their list of most diabolical video game villains of all time.