Diablo created the action RPG as we know it. Blizzard released it in 1996: a town, a cathedral, and sixteen randomly generated dungeon levels going down toward the Lord of Terror himself. You pick a warrior, rogue or sorcerer, click on monsters until they die, and pick up the loot they drop — which is randomly generated too, so every run turns up something different.
The atmosphere is what people remember: the slow, dread-heavy pacing, Tristram's guitar theme, and the way the dungeon gets darker and more hostile the deeper you go. This browser port runs the original.
Yes — a browser port of Blizzard's 1996 game. The official version is also sold on GOG and Battle.net.
The rogue is the most forgiving for a first playthrough — her bow lets you fight at range. The warrior is durable but must fight up close; the sorcerer is powerful but fragile early on.
Yes — it's a 1996 game and runs comfortably in a browser with nothing to install.