Actually Diablo is a mash up of 2 ye olde games. Gauntlet, and Atic Atac. Gauntlet was an arcade top down ARPG classic and Atic Atac was a 3d esque wireframe exploring, castle game on the Spectrum that featured a different map depending on which player you chose. There's nothing original under the sun.
Brevik along with Condor actually were making a turn based RPG with Claymation animation techinques. Every other publisher they approached thought they were nuts and told them no way. Blizzard and Condor bumped into each other while working on console ports of a SF2 clone based on comic superheros and Condor asked Blizz to publish diablo, Blizz agreed and made them change their turn based claymation game into a real time Action 3d isometic game or no deal. It turned out so good they bought out Condor entirely.
Brevik and the Schaefer brothers resigned from Blizzard after Blizzard told them their MMO D3 was terrible and to change it or else. They handed in their resignations over this expecting Blizzard to back down. Blizzard didn't back down.Also Blizzard North was shut down for a reason, Brevik and other key developers left Blizzard North.