User blog comment:Hereticalthoughts/Why Solaire is not, never was, and never will be the Firstborn/@comment-69.200.240.11-20160405222759/@comment-5.66.40.253-20160417140944

Replying to anon here. The Sword-Spear item description says that it was the basis for all cross spears that would later be used by other dragon slayers. In addition to his hair/crown/armour description saying that it resembles that of the Great Lord, and his soul description calls him a 'god of war'. It's the first born.