Our father slept for one hundred centuries, and while he slept we slept. He was raised from his long sleep, and we emerged into a galaxy far removed from the one we had left behind ten millennia before. We had been destined for the Thirteenth Legion, the Ultramarines, to take part in the Great Scouring, and to resume our father's father's Great Crusade. But we were brought to Mars, early into our trials, after our initial surgeries, and subjected to our long sleep in the care of the martian priests. When we were finally woken many of our peers were no longer with us, and had instead been replaced by other aspirants and neophytes. We were taken by full brother Astartes, and trained in our specialisations, and raised to full battle brothers. And then we heard our father's call. We rose to arms, legion, his Unnumbered Sons, and followed him on his Indomitus Crusade.
As Unnumbered Sons we fought, and bled, and died beside our brothers and cousins. And then our father took some of us aside, those who wore the blue of the Ultramarines and bore his gene seed, and he told us we were to be a new Chapter. We would drive through the Cicatrix Maledictum and bring light to the Imperium Nihilus. Brother Captain Elis Rhelian was chosen to be first among Captains, our new Chapter Master. He said that we would be Unnumbered Sons no longer, so we took a number—the number of our parent legion, and we became known as the Thirteenth Sons. For our icon we took the chevron of the Unnumbered Sons, but made it black on burnished steel. Our greaves we painted black to represent our new path, striding into darkness. And like a bolt our father fired us at the heart of chaos, and with violence and determination we carved a way through the rift, and so our crusade continues.
The Thirteenth Sons are a Loyalist Space Marine Chapter comprised entirely of Primaris Space Marines, created from the lineage of the Ultramarines and raised during the Ultima Founding of ca. 999.M41.
The Thirteenth Sons are a fleet-based chapter, and their task is to crusade through the Imperium Nihilus, bringing the Emperor's light to dark corners of the galaxy.
In addition to slaying the Emperor's enemies, the Thirteenth Sons diligently seek a method of reconnecting with the hundreds of thousands of worlds cut off from Terra in the galactic north of the Imperium Nihilus.
As a crusading chapter cut off from Holy Terra and without supply lines the Thirteenth Sons rely on captured and scavenged supplies. They have accumulated a large number of relics and trophies for a recently-founded chapter—many senior and veteran battle brothers sport helmets from earlier marks of power armour.
In a slight deviation from the Codex Astartes, squads of the Thirteenth Sons include a new rank—the Corporal acts as second-in-command to the Sergeant, and takes command of the second combat squad when that deployment strategy is used.