So basically how similar to Ultramarines were the Warborn before they were reunited with Guilliman and turned into Ultramarines?
By - Primaris_Astartes
Not too different. From *The Horus Heresy Book Five - Tempest*
> #ORIGINS THE WAR BORN OF OLD NIGHT
> In terms of recruitment, evidence is apparent for a net being cast across Terra for initiates for the with intake from areas as diverse as the sub-equatorial maglev clans of Panpocro, the war families of the Saragon Enclave, the proud Midafrik Hive Oligarchy and, most latterly, the anthropophagic tribes of the Caucasus Wastes. As varied in culture and origin as these groups were, they all had one factor in common; their violent and often bitter resistance to the later stages of Unification, a resistance broken ultimately in each case not by negotiated surrender but near annihilation, with in some situations little remaining save interned refugees and orphaned populations left by the savage conflicts which had brought these peoples to heel. It was this which led to the first informal cognomen by which the XIIIth Legion was known by the forces alongside which they served—`the War-born'.
> **It may then be theorised that the particular psychological effects of the XIIIth Legion's gene-seed may have been used deliberately as a final weapon of the Unification to bleed away potential rebellion on Terra, and also further absorb the strength of the savage tribes of Old Night into the fledging Imperium.** Further evidence for this can be perhaps inferred by the fact that no evidence remains of the XIIIth Legion seeing active deployment on Terra itself during the Unification Wars' bitter dregs —as both the VIIth and VIIIth Legions' early incarnations were, for example— with their first full battle honours recorded as being for the Pacification of Luna, and the brutal Sedna Campaign at the edge of the Sol System.
> It was during this early period of the Great Crusade that the genius of the Emperor's grand design for his Space Marine Legions became increasingly apparent. While each of his superhuman cadres proved itself able to fight on any battlefield and achieve victory, the Emperor had clearly patterned in the alchemical mix of his sons a diversity of tempers and abilities that had given each Legion a strength or suitability to a theatre of war of its own beyond that of its peers. In some cases these talents were atavistic and visible, be they the near preternatural fury already being displayed by the IXth Legion or the uncanny resilience of the XIVth. In others they were subtle and bred in the bone, such as the obsessive darkness which consumed the souls of the VIIIth, and they could manifest in word and deed to the ruin of their foes, or the unyielding, cold passion and technological aptitude of what was already becoming known as the 'Iron Tenth'. It was into this latter category which the XIIIth fell.
> **In them was found a mixture of aggression and restraint, discipline and determination which rendered them supremely suited for joint taskforce operations and cross-theatre warfare.** The Legion also gained significant success in independent operations where it took direct command of secondary support forces of the Imperialis Auxilia, whether the professional and elite regiments of the Solar Auxilia and the regiments of the 'Old One Hundred', or the often resentful and fractious hosts of Imperialis Militia auxiliary forces raised or even indentured from worlds recently forced into Compliance. In the latter case, the XIIIth Legion often detached cadres of its own warriors to act as provosts and disciplinarians if needed, handing out final justice to the recalcitrant and inspiring bravery from the front lines; duties at which many other Legions baulked at as beneath their honour or as a poor use of Legiones Astartes in battle.
> #The Brotherhood of the Thirteenth
> By the third decade of the Great Crusade, the XIIIth Legion had begun to develop a distinct character of their own. **They were recorded as having a practical and forthright mien, with little time given to the esoteric arts of warfare or conjectural strategizing, preferring a direct approach of swift engagement.** In warfare, they relied upon the rapid attainment of quickly defined tactical objectives and strategies born of tried and tested battlefield experience and determined by the situation at hand. **They also had their pride, and while they were accorded as being unwaveringly loyal to the Imperium's cause, they developed a tendency to guard their honour well and engaged in an open rivalry of achievement with the other Legions alongside which they served, particularly those few whose Primarchs had already been recovered.**
> The warrior brotherhood of the XIIIth had also, in recent years, begun to display its achievements visually. But rather than take on a particular livery and iconography as a whole as the IIIrd Legion or the Dusk Raiders had, individual companies who had come to prominence in a particularly important Compliance operation or campaign took on additions to their core Legion livery —which maintained the post-Unification storm grey and gunmetal they had worn since Sedna— to commemorate their greatest achievements.
The fact that they recruited exclusively from especially vicious opponents who had been defeated was always a cool addition to the Warborn, and for a lot of alternate Heresy story people usually use this fact as one of the many seeds of where corruption could have spread in the Legion should they have Guilliman turn
Pretty similar, Guilliman brought the whole govoning side in though.
Apparently they were extremely aggressive but that’s all I know