Diablo was far from the first ARPG, but given how influential it has been for the genre, I was floored to learn that it was actually designed and implemented as a turn-based game, and only turned into an Action RPG by request of folks at the main Blizzard office.
The founder of Blizzard North (David Brevik) thought it was a dumb idea, and only agreed to do it because it seemed like a large enough work item to justify requesting an additional budget milestone from Blizzard, which his office was hurting for sorely.
Brevik then got it working over the course of roughly an afternoon, by just running the turn system automatically and responding to clicks a little differently. It was only after seeing the action-oriented click-walk-attack-click flow actually work for the first time that he realized: they'd struck gold.
He told this story (and lots of others!) in a pretty excellent post mortem at GDC[1] a few years ago.