Unangband is an Angband variant which adds many, many new features, including a new terrain system (borrowed by NPPAngband), room descriptions, a wilderness/town system, new random artifacts, and much more besides. Unangband is more difficult than Angband but has a relatively 'deep' system of interaction between the various features in the game that players are encouraged to take advantage of. For instance, it is possible to use potions by setting them in traps, to coat edged weapons and arrows, to throw as grenades and to detonate them remotely in addition to the usual Angband method of quaffing them. A detonating potion might then set fire to other items or terrain adjacent to it, creating clouds of smoke which hinder line of sight and damage creatures in their midst, and so on.
Unangband has been referred to as a rough gem, in that features are often implemented ahead of game-balance and bug testing considerations. Recent releases have been much more stable, and the 0.6.3a release is playable and featured as competition 70 in the Angband competition. The author has made recent efforts to improve out of date documentation and to improve the overall start-up experience, particularly for non-Angband players, and to try to expand the audience for the game.
Unangband is so named because the author couldn't come up with an appropriate name for the variant. The Angband maintainer at the time, Robert Reuhlmann, called the variant Unangband temporarily and the name stuck.