So the lore of this would be they were part of the titan legion, fire wasps, during the great crusade, and I was going to say they were traveling with the Dark Angels primarily, as the fire wasps did, The reason for this, is so far what I have read in the Horus Heresy series, there is a lot of room for dark angels since they dont seem to have the spot light much, and I could say my titan legion originated from the fire wasps traveling with them, additionally I mainly paint dark angels. During their crusade, they formed a strong bond with the Dark Angels, Going even as far to petition Terra and Mars to allow them to rename them selves "The Guardians of Caliban." After the Death of Horus, they began their return to Caliban with the Dark Angels. Upon exiting the warp over Caliban they immediately, came under fire from the traitors on the surface. During the initial volley from the planetary defenses, the ships containing the titan legion were badly damaged, and in the retreat, their ship was deemed lost to the warp.

In actuality, the sudden need to retreat from the planetary space around Caliban, proved to be to taxing on their Astropath, killing them and causing a unplanned warp jump, putting them well away from Caliban. With vox and long range vox transmissions badly damaged, they did the only thing they could and limped their way toward Caliban in hopes to return with the fleet.

Upon arrival, they discovered themselves to be to late and the fighting had been finished and Caliban destroyed. Unsure of the fate of their befriended legion, the entire crew and members of the titan legion fell into deep despair, and mournful sorrow. The power to the engines of their ships were cut, and the ship was left floating adrift amidst the wreckage of the surrogate homeworld. They remained this way for several weeks, crews and staff fully demoralized, and barely able to maintain ship functionality. Scouting, search and rescue, along with salvage teams were deployed, more for protocol then an actual chance of finding anything. Their salvage teams managed to reestablish full ship functionality from before the engagement over Caliban. During their search, one salvage team recovered an item that spoke to them on a deeper level, it was a monument from their surrogate world. A masterful crafted statue of two watchers in the dark, creatures from Caliban myths holding up a golden ring. The watchers, which represented the Death and Mystery, and the ring, a symbol of hope, faith, and the spiral of life that never ended, a famed teaching of the knight of Caliban.

A larger version of that which is mounted on the back of the ravening champion bikes.

With the sight of this monument aboard their gak, in the presence of mighty titans, powered down, and almost bowing to it, the captain and the crew found a new vigor, and restored hope. As in at the drop of a hat, and the crack of a gun, the crew returned to life. It was ordered immediately to rid the titans of all former heraldry of the fire wasp titan legion from their titans. They were to be reborn as a new Titan Legion, "Afferte Ad Lucem" High Gothic for Bringers of the light. The former Heraldry of the fire wasps was jettisoned from the ship, along with all markings and records of their previous history, it was then littered with incendiary and small arms fire, and scattered among the debris, to give the impression they had fallen with Caliban.

With a new name, and new purpose, the legion became more akin to the free blades, a legion with out a home, or house, on the tails of their befriended legion, aiding them and all forces of the Imperium they come across, striding into battle with the coming of dusk, and providing hope for the defenders in the dark, and rewarding their faith in the coming dawn with the Light barer, and the wrath of the new day.