Considered by many to be the father of the modern roguelike, Angband was inspired by the original Rogue, Moria & Nethack. It has a plethora of variants available, some of which offer only slight changes in gameplay, and others which have completely changed the look and feel, developing into full-fledged games of their own like Tales of Maj'Eyjal.
The game sees you, a hero, fighting your way down to the 100th floor of the Pits of Angband in order to defeat Morgoth. The game world is based on the writings of J.R.R. Tolkien, and features many characters from his books. The dungeon layout and monsters are procedurally generated and different every time you change floor.
Zangband & Hengband are two variants you should definitely try once you've finished Angband itself.
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Pros
Pro Active development
Angband contributors are still improving the core gameplay almost 30 years later.
Pro Classic Roguelike that has inspired many variants
Throughout the years many people have created their own variants on the game. Making for a wide selection of choices in gameplay and themes while still holding to the core design of the game.
Pro Challenging
Angband is a very difficult roguelike game with permadeath, meaning that all your progress is destroyed when you die. Most Angband players don't fully beat the game until months or years later (if at all). However, the game still tracks your progress and scores even if you don't win, so it's fun and challenging regardless.
Pro Crossplatform
Linux / Mac / Windows support.
Pro Addicting
Pro High replay value
Each play through has the potential to be a different experience.
Pro Simple character creation
Pro Vast community of players
You can easily find tips and help online, about monsters, items, gameplay, etc...