One of my favorite games of all time is definitely an obscure one.
AngbandAngband is a direct descendant of Rogue, the original Roguelike.
A game called Moria was based on Rogue, and Angband is based on Moria. It's a fantastic turn-based adventure RPG. Everything is completely randomized, so you'll never find the same rooms or treasure twice. Gotta be smart though, as death is permanent! If you die, you can't reload (unless you "cheat" and save a backup file). Don't be fooled by its simple appearance though, this game is incredibly engaging, with a wealth of ways to play and survive. If you enjoy games like Dark Souls or Diablo, high risk high reward types, you'll love this game.
It's available for free on lots of different systems.