It's a freeware computer dungeon exploration game based (loosely) on the books of J.R.R.Tolkien. You explore a very deep dungeon, kill monsters, try to equip yourself with the best weapons and armor you can find, and finally face Morgoth - "The Dark Enemy".
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Angband was originally text-based, but graphical versions are now available for almost every computer system. Some Angband players still use the old ASCII-based graphics. There are even a few sound effects available for multimedia freaks ;-).