It's also on GoG https://gog-games.to/game/diablo and Internet Archive https://archive.org/details/Diablo_1996_Blizzard
To play the source port all you need to do is copy some files from the retail into the folder and it's good to go. It's also got quality of life changes like monster health bars, widescreen support, and a stash that transfers across characters. There's also multiplayer for classic and hellfire!
My headcanon for D1 is that you're not literally descending multiple levels. It's more about your sanity deteriorating thanks to Diablo's machinations so that you're hallucinating the demons and lava in the later levels. When you're killing Diablo, you're not actually killing a big red demon but wailing on some kid who Diablo possessed.
Of course, the headcanon makes no sense with D2, but it's still fun to think about.
That's a fun headcanon ngl