The XIth Primarch
Number XI crashed into an ocean world, risking being drowned, he was saved by sea hunters and been raised by them. Living in a solidarity society of both humans and aliens, XI didn’t take much time to become the king of hunters. Later XI was found by the God Emperor and accepted to serve the Imperium of Mankind but not for a long, when he was accused of heresy for supporting a Craftworld in a conflict long forgotten, he and his legion were cast out from the Imperium by his father the Emperor to be forgotten just like his brother the Second.
Now thousands of light years away from the Imperium, XI fights his way through with his legion allied with the Eldars.
This is my attempt to create a concept for one of the Unknown Primarchs from the universe of Warhammer 40k.