Ultima Underworld (1993), Ultima Underworld II (1994), Ultima VII (1994), and The Elder Scrolls: Arena (1994) all come to mind quickly as computer-based action RPGs that preceded Diablo (1996). And the Japanese were making top-down action RPGs as early as the '80s, especially those made by Nihon Falcom (Ys) and Quintet (Soul Blazer). I would particularly cite Brandish (1991, Nihon Falcom) as a similar type of realtime action dungeon crawler (although it wasn't released in the West until 1995).
All that isn't to diminish the substantial achievements of Diablo, which was quite innovative. The fast, smooth gameplay was novel, as was the setting. It was compulsively playable and accessible while a lot of earlier action RPGs were kind of clunky. The art design was superb -- everything from the dreary, gothic environs to the satisfying animation and sound of a pile of gold bursting forth from a slain enemy. It's fair to say that Diablo was a milestone in ARPG history and highly influential.
I'll disagree on all those (and the Ultima's etc in the grandparent comment). They were all focused on RPG and story first, then adding some action on top of that (via isometric, fps etc. modes).
Diablo changed things in that it focused on action first, whereas the RPG element is just an add-on. Its gameplay cycles between essentially going deeper in a dungeon that's becoming more difficult, obtaining/selling etc. of items/potions in town so that you can descend further. Its story isn't really of (heavy) importance.
All that isn't to diminish the substantial achievements of Diablo, which was quite innovative. The fast, smooth gameplay was novel, as was the setting. It was compulsively playable and accessible while a lot of earlier action RPGs were kind of clunky. The art design was superb -- everything from the dreary, gothic environs to the satisfying animation and sound of a pile of gold bursting forth from a slain enemy. It's fair to say that Diablo was a milestone in ARPG history and highly influential.