The 3rd company of the Ultramarines has been lost in the Koryndil Reach.
The last mention of the 3rd company in the novels (in Of Honour and Iron (Space Marine Conquest Book 4)) is of a great defeat in the Koryndil Reach, where the entire company is assumed lost.
A group of old Firstborns, that served with Fabian in the past were promoted to the First company at various points in time before the Korondyl expediton and they were thus spared the 3rd’s grisly end.
The Veterans built for themselves a reputation in the 1st for their extensive knowledge of Xenos, gained during the Tyrannic Wars of old; in particular Lucius Vorenus, first among them in both martial aptitude and leadership.
Once word spread that the 3rd was lost, they petitioned with Calgar and Guilliman to refound it.
Ultramarines leadership was unconvinced about breaking apart an existing company of Firstborns to fill the 3rd’s ranks.
Guilliman thus decreed that if the Veterans accepted to cross the Rubicon, and survived, he would draw the best of the best among the Unnumbered Sons. to backfill the ranks of the newly minted 3rd.
Thus the 3rd company would be re-made a full Primaris company like many others the Primarch employed in his crusade.
The company ultimately coalesced around two very different leaders and the diverse groups of battle brothers they brought with them.
Lucius Vorenus
Lucius Vorenus, a veteran of the Sternguard that fought Hive Fleet Behemoth in centuries past, accepted Guilliman’s bargain first and, after the transition, was invested Captain of the reformed company.
Amnis Transitores
With Vorenus at the helm, many more Veterans decided to cross the Rubicon and form the demi-company of the “Amnis Transitores” (the “river striders”), and seed the 3rd company a-new.
However, while unmatched in their skill and experience, the Transitores are not sufficient in number to bring the 3rd to full strength.
Publius Laurentius
Publius Laurentius awakened from his Ten Thousand Years slumber to a galaxy in flames. Born into a peripheral word of Ultramar shortly before Horus' betrayal, he spent a few precious years as a human before being put in Belisarius Cawl’s care.
In the succeeding millennias, Laurentius only experienced brief flashes of awareness as his body was turned into a weapon, spending most of his watch in stasis.
Then, the call arrived to join the second greatest crusade mankind had ever seen, Indomitus.
During one of his first campaigns against the revolting Xeno threat of the Necrons' Nihilakh Dynasty, brothers clad in black armor noted his fury in fighting the evil aliens and, quietly, an order was given that he would lend his sword to the honoured service of the Inquisition; joining the ranks of the Ordo Xenos' own Chamber Militant: the Deathwatch.
After 40 blood-stained years as the last bastion of Humanity against the unknown, the alien, the un-human: Laurentius has now been called home to Ultramar.
He brought great honor to the Sons of Guilliman during his service with the brothers in black, attaining the rank of Lieutenant and distinguishing himself for both valor and fury.
A different duty awaits him now.
Roboute Guilliman, in his infinite wisdom, has elected to nominate him Equerry to the Lord-Commander of the Imperium of Mankind himself. Reserved in antiquity for statesmen and leaders among the Adeptus Astartes, this new iteration of the role resembles more that of confidant and advisor.
Laurentius is thus elevated to the rank of Captain as befits one with such mandate.
Crusade Leadership is however not the duty the Primarch has in mind for him.
Laurentius is to join the newly re-founded Ultramarines 3rd Company.
He will serve as right-hand to Lucius Vorenus, a Firstborn brother from Macragge that “crossed the rubicon” to receive the physical might required to match his nigh-endless battle experience and unflinching hatred for the aliens.
Vorenus has been tasked to rebuild the 3rd as Ultramar’s speartip against the Xenos, and needs a trusted advisor.
The ranks of the 3rd include some of the most experienced warriors the 500 worlds can muster. All, like Vorenus, born in the last millennia and having only crossed the rubicon in recent decades.
The company, however, is anything but fully formed.
Laurentius' call is to aid Vorenus in integrating Unnumbered Sons and former Firstborn battle brothers in a single, cohesive company.
Drawing upon his experience in the Deathwatch, Laurentius makes some discrete inquiries into how his recall from the Chamber Militant came to be.
With that knowledge, he prepares a list of Grey Shields in active duty with the Ordo Xenos that would be prime candidates for the remaining posts in the new company and makes the necessary arrangements.
Over time, the Greyshields recruited by Laurentius coalesce into another demi-company with a common history and combat aptitude.
Torvum Velites
Intended as the younger, close quarter combat front-line of the company. All Primaris Astartes.
Born on some of the original 500 worlds of Ultramar during the last days of the Great Crusade and woken up during the opening of the Great Rift. Many of their leaders share a common fate with Publius Laurentius and spent the initial decades of the Indomitus Era in the Deathwatch.
They leave long-range fighting to their Rubicon brothers of the Transitores and instead face the Xenos threat head-on.
Their preference for melee combat, however, exacts a heavy toll on their ranks, as they suffer the most casualties against fangs, blades and claws of the enemy.
This leads them to develop mores and traditions that veer further away from the Codex Astartes than any other Ultramarines force.
Frequently, they claim tokens and trophies from their enemies and carry mementos of their lost brothers.
Their ferocity is unparalleled, their conviction unshakeable.
Strikeforce Advenarum Venatores
Together, these bulwarks of the Imperium carry forward Mikael Fabian’s legacy of Unflinching Hate for Xenos and Aberrations alike and set forth on their mission to give Humanity one more chance against the endless unknown.
Supported by others in the Ultramarines that share their vision of Xenos, they often intervene as the “Advenarum Venatores” strikeforce, hunting xenos and dismantling threats to Humanity everywhere they are needed.