to the Doom engine, initially under a restrictive license. On October 3, 1999, it was released again under the terms of the GNU GPL-2.0-or-later. With the source code available, it became possible for programmers to modify any aspect of the game, remove technical limitations and bugs, and add entirely new features. These engine modifications, or Doom source ports, have since become the target for much of the WAD editing activity, and with the decline of MS-DOS, using a source port became the only feasible way to play Doom for most people. Several source ports are in active development, and Doom retains a strong following of WAD creators. Types of WADs Levels The most common type of WAD consists of a single level, usually retaining the theme of the original game, but possibly including new music and some modified graphics to define a more distinctive setting or mood. Both single-player and deathmatch multiplayer levels are common. WADs may have a level pack in the form of an episode, replacing nine levels, and sometimes in the form of a megawad, which replaces 15 or more levels in the game (27 in Doom, 32 in Doom II, 36 in The Ultimate Doom). Total conversions A WAD that gives the game an overhaul to incorporate an entirely different game setting, character set, and story, instead of simply providing new levels or graphic changes, is called a total conversion. The phrase was coined by Justin Fisher, as part of the title of Aliens TC, or Aliens Total Conversion. Add-ons that provide extensive changes to a similar degree but retain distinctive parts or characteristics of the original games, such as characters or weapons, are often by extension called partial conversions. List of WADs The following is a select listing of popular and historically significant WADs. Megawads Bloom is a Doom II and Blood crossover released by Bloom Team in 2021. It features over 50 new enemy types, a new episode and an original soundtrack. Eternal Doom is a 32-level megawad for Doom II created by Team Eternal and TeamTNT. It was released non-commercially in several versions, with the final one being released on November 14, 1997. Eternal Doom received media attention in 2020, when through further modding, it was played through Doom Eternal. Going Down is a 32-level megawad for Doom II released in 2013, and a winner in that year's Cacowards. It was created by