Sil is one of the most popular variants among non-Angband players, as it does not suffer from the same prejudices regarding being overly long. It is a re-imagining of Angband, set during the First Age, aiming to be as faithful as possible to the source material. The player has the same task as Beren in the Silmarillion - to steal a Silmaril from Morgoth's crown, although need not be and usually isn't human.
The best introduction to Sil is the extensive manual distributed with the game. To summarise here; a total reversal of power creep was achieved by paring down and reworking the game mechanics, with an entirely new set of opponents. Many of the staples of other variants are gone with the removal of almost all explicit magic besides song. One huge consequence to the gameplay is that teleportation is forbidden and allowing yourself to be surrounded is correspondingly more likely to prove fatal. Character progression is based on skill levels which may be purchased with experience points, rather than character levels which are completely removed. Experience can be gained through merely encountering enemies, descending to lower depths and identifying items, but experience gains fall off rapidly for killing or encountering the same type of monster over and over again. Besides skills, which are numerical enhancements, there is also an extensive ability tree which can modify the behaviour of the character, by allowing e.g. attacking and moving in a single turn. These factors all blend into a variant that is both strategically and tactically interesting, providing shorter games than most variants while ramping up quickly in intensity.