Lost and the Damned
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A General term for the various cultists, pirates, mutants and so forth that are not space marines and fight for chaos. Well known for fantacism, narm, disposibility (c'mon, they're evil Guardsmen), and alternating between complete brilliance and brain-shattering idiocy depending on the individual force group; some, such as the Blood Pact, Children of Nemeroth, and Brethren of Fire, are well-trained and organized, led by actual Chaos Champions, whereas others are traditional Chaos-worshipping pants-on-head retards. Most Lost and the Damned force groups are considered at least 50% as ballsy as their Imperial counterparts, which is fairly ballsy by the standards of the 41st Millenium.
In general, Lost and the Damned forces played somewhat similar to the Imperial Guard, but with a number of unique differences. Most notable is that they got different heavy and special weapons, a few different vehicles (including the option of Rhino Transports OR Chimera Personnel Carriers for transports, Defilers AND Leman Russ Battle Tanks for fire support, and so on), and that they replace the usual suite of abhuman and specialist units the Guard have - Ratlings, Ogryns, and Veteran Squads and Storm Troopers - with versatile units like Mutants, Big Mutants, summoned Daemons, and Chaos Space Marine squads. The result was a tactically-flexible army, but one that traded the raw firepower of the Imperial Guard Codex of the day for flexibility and genuinely fun-to-use options.
As of today, the Lost and the Damned are both hopelessly outdated and no longer supported by Games Workshop, functionally making them our generation's Squats. Despite popular demand, there's no indication that the army will ever be re-released, even though Forge World still carries models for it. Sadly due to the lack of the proper codex, as of thus far, you can only use them if you count them as a Guard Army, which may or may not be even possible depending on what your force group originally consisted of. If it had a lot of Daemons, Zombies, or Mutants congrats: You're fucked.
Fucking Games Workshop
The Lost and the Damned army list was a hell of a lot of fun, and was designed for those who had scored Codex: Eye of Terror for campaigns. The idea was simple: an army full of non-Marines dedicated to Chaos - pirates and raiders and cultists and mortal warriors devoted to Chaos, all under one banner, albeit with Chaos Marines allowable as Elites (and you could even get a cheaper, if weaker, Chaos Lord/Sorcerer as an HQ).
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The combination of lots of modding possibility, a potentially characterful army, and some interesting ideas to differentiate it from the Imperial Guard went a long way towards endearing it to players, since the idea of cultist units had been toyed with before in the previous Chaos Codex (with the Alpha Legion. This was taking it to its logical conclusion, and suffice to say, some people thought this was pretty cool. Forge World saw potential here as well, and to promote the new army list, put out some bad-ass new resin-casted models. Several people started to get into the new army, and there was a lot of Derp and win as players used this to put out some truly entertaining army lists, from Zombie Apocalypse setups backed with heavy armor, to squads of Traitors backed by Mutants and APCs.
True to form, however, the second that the official tournaments of the season ended, Games Workshop abruptly stopped supporting the army entirely, refused to allow Lost and the Damned armies into future tournaments, pretending the previously-supported army didn't exist, and left players with naught but the Counts As rule to keep them company - and then there was RAGE. A few fortunate souls - the ones focusing on infantry and not-Chaos armor - could get away with using their killy and flash models as a viable Imperial Guard army, but these players were a distinct minority, since a lot of players had used the rules to field more versatile, interesting, or outlandish lists, especially given how expensive the models from Forge World are.
Fuck you, GW.
Promotions
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A few fa/tg/uys who infiltrated GW managed to put out new Lost and the Damned rules for the Tempus Fugitive events, using the newer Chaos Space Marines and Chaos Daemons codexes. Sadly, due to GW not giving two shits, it's unlikely that it'll ever be made legal outside of these events, but anyone who wants to take a look can find revised rules below: