NetHack

NetHack is a dungeon crawl from 1987 that people are still finishing for the first time. Every level is generated as you walk into it, everything in the game interacts with everything else, and death is permanent — it is a game meant to be lost, repeatedly.

The demo drops you in as Ada, a human Valkyrie with a long sword and a pet kitten. h j k l or the arrow keys walk, walking into a monster attacks it, , picks up whatever you are standing on, i is your inventory, and > takes the stairs down once you find them. The VM is private to your browser tab; nothing you type leaves it.