Mortarion

After being thrown through a warp portal from the laboratories of his father, the Emperor of Mankind, little Mortarion crashed inside his capsule on Barbarus, the worst planet in the galaxy to live on. There lived horrible, detestable people, the worst of the worst in the galaxy, necro-lords with the psychic ability and intelligence to create horrible plagues and cause frightening torture. The most important of them was the one who found the little Primarch. Seeing that he could resist the noxious gases of a certain part of the mountain without protection, he decided to adopt him.
Mortarion was raised by a crazy man, who mistreated him, hit him and humiliated him whenever he could, nothing Mortarion did was ever right and he would never meet his adoptive father's expectations, he would always be a disappointment and could never decide his destiny.

After meeting Typhus who was a child about to be killed by followers of his father, this boy asked a young and mature Mortarion to save him, both managed to escape and start the greatest revolution in the history of the world of Barbarus, over time they defeated with the help of civilians all the cruel lords, but the more their campaign advanced fewer people could follow them, the damned were hiding in the mountain of the world that the higher you go the more toxic its air is, when his adoptive father was left alone and being the only one who could rescue the toxic gases from the top of the mountain Mortarion went to confront him, confrontation in which he was knocked out and upon returning to have consciousness his genetic father the Emperor killed the man who caused him so much misery, but Mortarion far from being happy despised his father since in his words "You stole my revenge, you stole my freedom" and that was how Mortarion had to leave Barbarus to join his father's Empire and command his legion

Mortarion did not get along particularly well with all of his brothers, but Horus was someone that Mortarion at least respected, when he was named Warmaster was that Mortarion knew that great things awaited him, when the time of Horus' betrayal came Mortarion was one of the first to join him, once he knew that part of his legion would remain loyal to the Imperium and who not the loyalists of the Sons of Horus, the Emperor's Sons, the World Eaters and the Death Guard were sent to a "battle" after this event and the entire Horus Herjia Mortarion ended up as a daemon prince of Nurgle, once again serving someone he despised and powerless his only consolation was there in the warp the soul of his adoptive father which to this day he continues to hold captive and torture whenever he has free time to do so *

He wears heavy armour but is still very fast compared to anyone who is not a Primarch, he uses as a weapon a large scythe the size of an Adeptus Astartes and respirators with two green tubes that give him an even more emaciated appearance.
His armour is thick and heavy and due to his experiences in Barbarus Mortarion can withstand an unreal amount of damage to his body and continue fighting* like every primarch he is an absolute genius with an exceptional brain, a great warrior, a natural leader and he emanates a poisonous aura that withers the plants around him, he can use some of Nurgle's plagues and infections to infect his enemies instead of just splitting them in half with his scythe.