Slash'EM Extended (also known as "Sadistic Levels of Endless X-Citement", or SLEX for short) is a variant of SLASH'EM developed by AmyBSOD and based on SLASH'EM version 0.0.7E7F2. The object of the game is the same: to fetch the Amulet of Yendor from the bottom of Gehennom and offer it to your god.
Compared to SLASH'EM, the aim of Slash'EM Extended is to be a "kitchen sink" style variant that heavily raises the difficulty of the dungeon and adds several new roles and races for the player to choose from, alongside several new special levels and a near-endless amount of monsters; the developer is also known to deliberately implement content from generally-obscure and lesser known variants (e.g., a dungeon branch dedicated to ZAPM) as well as Evil Patch Ideas and Evil Variant Sokoban levels.
However, due to some of the new content being highly inappropriate, or in some cases illegal depending on location, most of the information regarding it has been removed – this page exists for historical purposes. The repository for Slash'EM Extended can be found on the GitHub website, but access to the repo comes with an 'objectionable content' warning banner, and an account is required to view the content. Some of the less-objectionable content has influenced features in other variants of NetHack: particular examples include in-game monster lookup, which has since seen use in many variants, and the idea of various monster "templates" that can be applied to randomly generated monsters (which exists in SLEX as "egotypes").
SlashTHEM in particular was originally a direct successor variant of Slash'EM Extended v75, and eventually diverged significantly in favor of becoming a more SLASH'EM-based fork - it also retains and builds on many of the elements originating from early Slash'EM Extended and the other variants it drew from, including NetHack: The Next Generation, SLASH 6 and NetHack-- 3.0.10. Remnants of influences specific to Slash'EM Extended include the Acid Mage and Electric Mage roles; the article on SlashTHEM has a list of additions to the game and their sources, and this article has a list of features from Slash'EM Extended that still exist as deferred in SlashTHEM.