Dungeon Crawl Stone Soup, better known as DCSS. https://crawl.develz.org/

The premise is simple: Dive into the dungeon, retrieve the famed Orb Of Zot, and abscond with it in your possession. Easy, yes?

What sold me were the vi-style controls, the ability to play directly from a terminal (or a prettier web GUI if you so choose), and how every action is on a turn-by-turn basis. You get to pause/stop at any time, and your progress remains for years.

It can be a little daunting to get into it, but it is a lot of fun!