Oh look, an old game that's appropriate for Halloween!
Diablo is a classic Blizzard game. The setting is fairly simple: there's a church (monastery?) that has been overrun by demons. You (the hero) must go into it, figure out, and stop what is going on there. There's a huge problem: Hell has somehow broken through and emerged into this church. You start going down through the tunnels, past the mauseleums with skeletons and zombies. Eventually you reach some catacombs and caves filled with some minor goat demons. Here and there are some bosses in some special areas. You finally reach Hell and its hordes of sorcerers. Gameplay heavily depends on the character type you've selected, but its standard RPG fare. You do things, level up, select what to be better at, collect things, and go off to save the world.
Because I have a tendency to bash things to death, I played the warrior. Unfortunately, the ranged enemies do not stay still when you run up to them. They have a tendency to scatter, and one will lead you on a wild goose chase straight down an unexplored hallway. I didn't notice until I was about done that you can buy potions that increase base stats without leveling up. Whenever I got enough gold to completely fill an inventory spot, that should have told me to go buy one of these potions.
I really dig this dark gothic artstyle that is common in 1990s Blizzard games. It evoked similar 'monster hiding under the bed' feelings that StarCraft did. Darkness and terror are palpable, and it's right there. Somewhere there's something unknown lurking in there that will completely mess you up!
At GDC 2016, David Brevik, co-founder of the studio that made it (later bought by Blizzard and renamed), said he would release the original design document for Diablo. I read it, and it gives good insight into what they wanted to make, and how much of it remained. He also did a full post-mortem, even though it's been 20 years.
I remember playing this about 15 years ago. I remember getting most of the way through (maybe to the last level or so), but decided that it was too hard. I thought that this would be a good game for Twentieth Century. I looked, and it surprised me that they are simply giving it away for free. Check it out!
And yes, when I started playing, it was exactly as I remembered it from all that long ago. I remember having just as much fun.