Marathag is a very new Craftworld, led by Ariel Ysggad and crew/inhabited by Eldar outcasts and exiles, trying to turn it into a 'proper' Craftworld while fighting off Chaos, Orks, their dark cousins, and Adeptus Mechanicus forces intent on, variously, capturing, destroying, and studying the Craftworld.
Almost nothing is known about Marathag's history before its recent discovery, but Navarre and Ariel Ysggad have theorized that at the moment of the Fall of the Eldar, the Craftworld was not yet complete; perhaps the result of a last-ditch effort of Eldar to escape. When the Fall came, Slaanesh was born, and the Eye of Terror was created; they believe that the Craftworld may have become pulled into the Warp in a manner similar to Imperial vessels traversing the Warp(as opposed to the Eldar Webway) and drifted in the Warp for thousands of years
All that is known for certain is that Marathag Craftworld was discovered by Chaos forces loyal to Slaanesh, including some of the Corpus Brethren, in 997.M41 floating in the void above the Krimzon system, having apparently come out from the Devil's Eye, a massive Warp rift nearby.
Before the Slaaneshi could take possession of it, a large fleet of Dark Eldar appeared and attacked them; Ariel Ysggad first came to Vigo to investigate increased Dark Eldar activity in the region, and he believes that the Dark Eldar somehow learned of its impending reappearance.
As the Slaaneshi and Dark Eldar fought, a large allied Imperial/Eldar fleet, led by Karlees Navarre and Ariel Ysggad arrived, took the two enemies by surprise, and routed them.
While the Eldar pursued the Dark Eldar, the Imperial forces secured the system and discovered what the Dark Eldar and Slaaneshi had been fighting over; Navarre and a small force of Stormtroopers teleported aboard, and discovered that not only was the Craftworld uninhabited, it had never been inhabited; it was then that Navarre started to formulate his theories regarding the Craftworld's origins.
To avoid war with the Eldar, Navarre invited them to take possession of it(much to the indignation of the Adeptus Mechanicaus when they learned about it) on the condition that the Craftworld would remain in the Vigo Sector for a thousand years, and remained allied with the Imperium for all that time; many Eldar commanders refused to accept and almost started the war anyway, but Ariel Ysggad preempted them, taking control of the Craftworld, naming it Marathag(Face of Death) for its empty, uninhabited state, crewing/populating it with his followers, and invited other exiles to join him.
Since then, Ariel and his people have slowly opened up much of the Craftworld to habitation, and slowly increased both their numbers and resources; and fought off many attempts to capture or destroy the Craftworld made by the Dark Eldar, Orks, Chaos, and the Adeptus Mechanicus.
Other Eldar, including forces from Biel-Tan, and Prince Yriel's Eldritch Raiders, have attempted to take Marathag; The Biel-Tan forces were turned away peacefully, but Ariel Ysggad had to call on Imperial forces to help him turn away Prince Yriel, when he tried to use force to take Marathag, resulting in a tense standoff; exactly how that was resolved peacefully, no human knows, but after a short private discussion between Ariel Ysggad and Prince Yriel, the prince gathered his fleet and departed, never to return.
Like all Craftworlds, Marathag contains a Webway gate - the Webway linking the craftworlds to each other as well as to millions of planets, allowing the dispersed Eldar civilization some measure of cohesion. Because the Webway is labyrinthine and impossible to map, many craftworlds are effectively lost, unknown and cut off from the greater part of Eldar civilization.
Marathag is a very small Craftworld; other Craftworlds, inhabited and built up since the fall, have grown greatly in size, up to 100 times bigger in volume than they were before the Fall.
Craftworlds are effectively worlds in space, each a self-contained biosystem, with forested and natural areas as well as urbanised ones. These natural areas provide a breathable atmosphere to the Craftworld and renewable resources; though there is plenty of vegetable life on Marathag, prior to the arrival of Ariel Ysggad and his followers, there was no animal life of any kind, which only deepens the mystery of this Craftworld's origins.
Most of Marathag is uninhabited and awaiting reconstruction.