Founded to replace the traitorous Iron Warriors as the IVth Legion, the Avenging Hawks were a Second Founding successor Legion to the Ultramarines. An extremely conservative Legion, the Avenging Hawks adhere rigidly to the principles laid down by Rogal Dorn in the Principia Belicosa, and were founded by those Ultramarines most distressed by the loss of their Primarch, Roboute Guilliman, at the hands of the agents of the nefarious Alpha Legion. Viewing Guilliman's death as the failing of the Primarch's sons to protect their gene-father, the Avenging Hawks believed that each of them had a personal responsibility to devote themselves to avenging their Primarch's death, and that each must carry the burden of Guilliman's death with them to the grave, repenting for the failure of their ancestors throughout their waking lives. As such, the Avenging Hawks were a ferocious and uncompromising Legion, caring little for frivolities such as politics or bureaucracy, being only concerned with meeting whatever foe threatens the domains of the God-Emperor and striking them down without mercy before continuing on to the next battle. The Hawks believed even a moment's respite from battle or from the preparation for battle to be unacceptable, for it was during such a moment of laxity that their beloved Primarch was struck down.
The Avenging Hawks were founded under the leadership of Aurax Centarius, a member of Roboute Guilliman's personal Invictus Guard. It was Aurax himself who meted out judgment upon Guilliman's murderer, and Aurax took personal responsibilty for Guilliman's death. In the wake of their Primarch's loss, the XIIIth Legion and the realm of Ultramar fell into a state of confusion and disorganization. Out of a sense of duty, Aurax, albeit reluctantly, took his gene-father's place, galvanizing the Legion to dispense retribution upon those whom had taken away the Ultramarine's beloved Primarch. Centarius brought order back to Ultramar, assuring its people that vengeance would be had for Guilliman's death. He then gathered together the XIIIth Legion's greatest warriors, taking them to the Segmentum Obscurus to dispense judgement upon the Traitor Legions still fleeing from Imperial Space.
Under Centarius' leadership, these champions of Ultramar cut a bloody swath through the retreating traitors, intent upon finding the former XXth Legion's Primarch. And they would, upon the world of Hydrast II, only for the Primarch to slip narrowly through their fingers. But Centarius' men were not fazed, and continued their hunt. And so they found Alpharius Omegon yet again, this time on Murdax Tertiarus, but again the Primarch would slip away. So began a long and bloody game of cat and mouse, with Centarius' champions searching and eventually finding their quarry, only for Omegon to slip between their fingers at the last moment. This continued until Centarius's twelfth encounter with Omegon, when the Primarch's right leg was crippled by a well-placed lascannon blast from one of Centarius' Devastators. Aurax would then corner his prey, using his Power Fist to pummel Alpharius until a gaping two foot wide hole was made in the Primarch's chest cavity, then leaving Omegon for dead. Satisfied with his work, Centarius and his remaining champions returned to Ultramar, where they were hailed as heroes. In honor of their deeds, Centarius and his surviving champions would be used as the foundation of a new IVth Legion.
Despite taking vengeance upon the Alpha Legion's Primarch, Centarius' guilt over Guilliman's death remained. As such, he founded his new Legion around further avenging the death of the Ultramarine's Primarch, to this end christening his Legion the Avenging Hawks. And so it was decided that the Avenging Hawks would pay eternal penance for the failures of Aurax and the failures of all those descended from Guilliman's genetic template, penance that would be paid in blood. Upon his Legion's homeworld of Chorda, fifty-thousand Astartes Legionnaires were brought to arms over a century's course, following the guidelines of Rogal Dorn's Principia Belicosa, a revision of Guilliman's unfinished Codex Astartes, a work that would've surely been the Primarch's magnum opus had it been able to have been seen through to completion. With these fifty-thousand Astartes at his disposal, Aurax would take his newly-constructed Legion with him back to the Segmentum Obscurus, to see out the rest of the Great Scouring. On worlds such as Ogridath, Lasnius Prime, Kolgatos and Eternust, the Hawks would quickly establish themselves as being frightfully effective, with warriors as skilled as they were brutal, and rightly feared.
As the atheistic Imperial Truth was slowly replaced by the superstitious, theocratic Imperial Cult, the Avenging Hawks were among the majority of Legions who accepted and enforced this radical shift in ideals within the Imperium, welcoming it, even. The war-mongering Hawks were more than happy to use so-called "heresy" as an excuse to wage even more war.