For ten thousand years from the [[Great Crusade]] until the end of the [[13th Black Crusade|Thirteenth Black Crusade]], the Firstborn were simply the [[Adeptus Astartes]]. However following the development of [[Primaris Space Marine]]s by [[Belisarius Cawl]] the "Firstborn" as they became known faced the prospect of obsolescence at these larger, physically stronger, and mass produced warriors.{{Fn|4}} Originally, in the early stages of the [[Indomitus Crusade]], most Primaris were produced by Cawl himself and, though developed by the [[gene-seed]] of the [[Primarch]]s, lacked a distinct identity, instead being gathered in armies known as the [[Unnumbered Sons]]; however, as the Crusade progressed, the Unnumbered Sons were distributed to either new [[Chapter]]s of the [[Ultima Founding]] or into existing Space Marine Chapters.{{Fn|2a}}

Even though 94% of [[Chapter]]s have accepted Primaris Marines into their ranks{{Fn|2}} Firstborn-Primaris relations have had issues. To many, the Primaris were new arrivals often recruited away from a Chapter's [[Adeptus Astartes Homeworld|homeworld]] and new to the culture, traditions, and ways of the Chapter. Many arrivals from the [[Unnumbered Sons]] into existing Chapters felt like strangers.{{Fn|2b}} When some Firstborn first saw the Primaris, they regarded them as mutants.{{Fn|4}} This tension was exemplified in the [[Space Wolves]], where many [[Wolf Lord]]s resisted accepting Primaris Space Marines.{{Fn|3}} While the likes of [[Krom Dragongaze]] embraced these new warriors, [[Logan Grimnar]] himself was distrustful and say these warriors not of Fenris as means to control the Wolves by Guilliman.{{Fn|11a}} However after experiencing the trials of [[Gaius (Unnumbered Sons)|Gaius]], Grimnar had a change of heart.{{Fn|11b}} In the [[White Scars]], many Firstborn saw that these warriors were not of [[Chogoris]] nor knew the ways of their Chapter's culture, but at the same time understood the necessity of Primaris Marines in defending the [[Imperium]].{{Fn|4}}