This is a thread for all of the "forgotten" armies of 40K. Simple really...
Harlequins are a mysterious sub-sect of the Eldar race. They are a combination of mystic, bard and warrior, waging an endless war against Chaos, and guarding the secrets of the Webway and their people. Their greatest secret is the Black Library, a mysterious place within the Webway that has been sought by the Imperium and Chaos with equal measure.
Harlequins represent a united form of the Eldar. They recruit from all walks of life, from the Exodites to the Kabals of Commoragh, and woe befall anyone who tries to refuse a Harlequin that which he desires...
Citadel Journal 39
Citadel Journal 44
Troll 25 rules
EO Harlequin Codex, 3rd Edition
EO Harlequin Codex, 4th Edition
Exodites are Eldar who abandoned the decadant life of their people before the Fall. They chose to live a simple life, often on harsh and inhospitable worlds (by Eldar standards!). They are known to make use of "dinosaurs" as mounts or monstrous living war-engines.
Whilst isolationary, the Exodites usually have ties with the Craftworlds or Eldritch Raiders. They rarely (if ever) deal with the Dark Eldar.
EO Exodite Codex (v1.0).
Exodite Online Codex
Agis Page Exodite Codex
Piratical Eldar who travel the stars in search of treasures, the Eldritch Raiders are not as cruel or sadistic as the Dark Eldar, but nor do they adhere to the disciplines of the Craftworlds. They usually hire their skills as mercenaries, and have connections with the Zoat race. Their true motives, like their ships, are elusive as starlight, and only a fool would trust them...
Eldar Online Codex.
Orks who have yet to learn "propa Orky kulture". They may be primitive, but they do know how to fight like an Ork!
WD Army List
Genestealers are a vanguard species of the Tyranids. They travel the stars on Space Hulks, massive derelicts that float through the Warp. How they board these ships is unknown, but the danger they pose is so great that entire Chapters have been commited to the task of hunting down and purging these vessels.
Unfortunately, they do not reach them all, or fail in their attempts. When this occurs, a Genestealer Cult will be formed. The population is corrupted by the invasive genes of the Genestealer, and Hybrids become abundant. When the Cult reaches critical mass, it swarms up in a tide of rebellion, consuming the world before infesting any available ships with their Purestrain Genestealers and loyal Hybrid guards, ready to continue the spread of this terrible evil...
Cult Codex from Inisfail
Another Cult Codex from Inisfail
Servants of the Omnissiah, the Adeptus Mechanicus are masters of technology within the Imperium. They command the mighty Titan Legions, and bionically-enhanced Skitarii, and their forces often contain some of the deadliest weapons mankind ever produced, such as the Ordinatus.
Whilst Adeptus Mechanicus armies are relatively rare, they do exist, and they do see battle. The Adepts may wage war to claim rediscovered technology, or to defend an outpost or Forgeworld.
Codex from Inisfail
The Arbites are the enforcers of Imperial law. Raised in the Schola Progenia, the Arbites bring a brutal and unforgiving brand of justice to their garisson-worlds. When rebellion occurs, the Arbites from the first, and sometimes only line of defence in the battle to maintain Imperial rule. They will return the world to the Emperor's light, or sell their lives dearly in the attempt...
Citadel Journal List
Codex: Book of Law part 1
Codex: Book of Law part 2
Codex: Book of Law part 3
Sometimes called "Halflings", "Hobbits" or less desirable names, Ratlings are an abhuman race, standing a little over three feet tall. They make excellent snipers, and are also known as thieving little blighters. Any Guard regiment that serves with the Ratlings had best check their kit regularly...
The Hrud are a little-known, but seemingly common race in the 40K universe. What few images that exist reveal them to be a manky, rat-like species that dwell within warrens. Thus many have concluded the Hrud were originally "space-skaven", and the theory has stuck.
Apparantly, the Slann made all the other 40K races. Little else is known, and we are thankful for that. For a race that can supposedly beat the Necrons inside out, you'd think they'd have a better list...
Codex: Slann from Inisfail
Squats were a race of abhumans, much like Ratlings or Ogryns. They lived in isolated worlds, forming "Brotherhoods" that treated war as a business, selling their technology and troops to the highest bidder. They were extremely talented inventors, and proficient technicians.
The Demiurg are widely viewed to be the new incarnation of the Squats. Dwelling aboard vast Commerce-ships, the Demiurg are described as short, and technologically skilled. Like the Squats, they too live in "Brotherhoods". Whether the bikers and leather will return is still unknown...
Squat Codex from Inisfail
Another Squat Codex from Inisfail
Squats as Orks
Codex: Karak by myself.
Giant insects from the planet Vespid, these critters form a powerful auxiliary within the Tau Empire... and, it seems, quite a nasty force of their own!
Unofficial Codex by Omeganus
Humans in the service of the Tau Empire, these lists will allow you to field your own armies of mankind in service of the Tau'va.