Little is know about the races origins other then what little the Eldar knew of them before their fall. The Cyber Harbingers may know more as they have been around since the galaxy was young but they are rather... Reserved. So meta-game history is required for this.
The Lost were originally a nomadic race never staying in one place to long always seeking out new places. Naturally curious and intelligent they were able to harness the power of FTL hundreds of years earlier in their history then humans and none to soon as a cosmic event caused the destruction of their home system the anomaly continued to spread engulfing colonies as it spread. Those who remained fled into the void deciding never to call any planet home. Thus began the great journey as millions of their people set out to explore the galaxy. However it was not to last as the Eldar began their doomed war with the inter-dimensional enemy known only as the Darkness the travelers fleets where hunted down and destroyed. Until only a small fraction remained. Utilizing "acquired" technology to open an inter-dimensional gate the travelers fled never to be heard from again, until now. There they where trapped within the void between universes, In the very realm that the Darkness called home.
Emerging from their inter-dimensional prison ten thousand years later the travelers now called themselves The Lost. The fleet was scattered by their exit and only a small number arrived in the area where the T.E.F's first contact occurred. The Lost ships immediately opened fire and destroyed the T.E.F taskforce and continued on their way. Battles have continued to increase in intensity with the most violent being the sacking of Caulston where the full might of The Lost collided with the 3rd Armada. the T.E.F force where annihilated and Caulston has been enslaved by The Lost. Recently it would appear that the Eldar and the Cybers have recognized the energy signature and fearing another attack from the Darkness have sent their fleets to investigate. The Eldar and Cybers have not only been fighting The Lost but also each other. The Sector is now engaged in one of the largest military engagements since the T.E.F's battles against the UCI during the rebellion. The Lost however continue their expansion as more and more arrive from the void it seems that soon they will have full control of the sector.
The original Travelers where a tall slender people with a blue colored skin, they had no hair. There was also little sexual dimorphism with females and males essentially being the same in terms of strength life span and appearance.
It is not known whether their appearance has changed as they are now only ever seen in their suits of armor, but many of their attributes have. For example gone is that natural curiosity and general warmth replaced now with a cold intelligence. They also have appeared to have had a massive boost in strength and stamina reports of one of their warriors taking on a cyber battle drone in hand to hand combat and lasting a good minute before being pounded into putty have amazed many (remember the drone is a 10 foot death machine) Not necessarily cruel they are merciless and without pity preforming mind altering surgery in their slaves to keep them loyal.