The native worlds influenced the formation of the Emperor's creations: Angron received a physical mutilation, Conrad Curz's mind was swallowed by darkness, Primarch Mortarion inherited both ailments. So describes the sullen commander Chris Wright in the book "Demonology."
Childhood
The kid was picked up by the war master and raised in captivity in the conditions of the toxic atmosphere of the planet Barbarus. Xenos found a baby who could breathe poison among the dead on the battlefield and named Mortarion ("son of death"). The child grew up and honed his acquired fighting skills and once escaped from captivity to find out who is hiding below. And the master of war moved to the highest peak of the planet, fearing revenge.
Arrival of the Emperor
The child met people in the lowlands and stayed, helped them fight against the oppressors, imparted knowledge of the art of war.
Over time, the "father" of Mortarion became the only undefeated enemy due to the inaccessibility of the mountain. The future primarch carefully prepared for the ascent, knowing that he was going to death.
The Emperor, who arrived at Barbaras, set the condition: having lost, Mortarion would join the Great Campaign.
For a young man, victory over the last master of war was the goal of life. When he climbed to the top, poisonous clouds corroded chemical protection and deprived him of strength. Over the implacable warrior the braid of his adoptive father ascended, but the Emperor defeated the xenos with one blow.
Primarch Mortarion admitted defeat and took command of the Death Guard legion, but secretly held a grudge, for he had lost his victory and the meaning of life.
Before betrayal
The intransigence, pragmatism and determination of the leader were transmitted to the soldiers.
In bringing the worlds to agreement, the XIV Legion used chemical and bacteriological weapons that left radiation deserts unsuitable for life.
The Death Guard specialized in sweeping worlds with a poisonous atmosphere unsuitable for landing other Astartes, and fought fierce battles with the Xeno-Race of the Yorgals.
After the conquest of another planet, a sample of the poison of the most dangerous creature was taken and drunk. In this way, fighters increased immunity. Mortarion carried a flask of poison with him: to drink with the primarch is a great honor.
Heresy
Corvus Corax and Robout Gilliman expressed concern when they saw how close the son of death was to the Warmaster, but not to the Emperor. In response to the warning, the Overlord of Humanity stated that loyalty to the Master of War means loyalty to the Emperor.
During the Civil War, Primarch Mortarion joined Horus of his own free will, as hostility towards the Emperor grew every year from their first meeting in Barbarus.
The Death Guard participated in the bombardment of Isstvan III with poison gas and in the massacre of Isstvan V, where the apostate legions dropped their masks, declaring an open war.
Rebirth
“ Accept my essence, Mortarion,
You don't need any more false crowns.
I will save the life of your Legion
Listen! Do not destroy my gift in the bud! ”
- Keepers of Death The Mortarion Saga
Despite the betrayal, the son of death held atheistic views. A turning point happened when the legion rushed to Terra: they fell into a warp storm, the navigators did not find a way out into real space, and the fleet anchored.
A swarm of plague flies infiltrated the ships of the fourteenth legion. Unbending immunity, able to withstand any infection, became a curse, turning soldiers into immortal mutants.
In desperation, Mortarion, stricken with illness, recalled climbing the poisonous mountain of Barbarus. The hopelessness and nearness of death clouded consciousness. There was no Emperor who saved at that time. Two paths remained: eternal torment or death as liberation. Mortarion grabbed onto the last spark of hope that flashed through his mind: to appeal to the warp entities of which he had heard legends. Grandfather Nurgle answered the prayer.
The primarch accepted the gift of Chaos, the legion was reborn. What came out of the warp was the diametrical opposite of what came in.
After heresy
After the defeat of Horus, the legion retreated to the Eye of Terror and occupied the Plague Planet. The son of death studied warp and collected information about psykers. In Chris Wright’s Demonology, he captured the demoness Tzincha Lermenta, who convinced him to engage in witchcraft.
In the book “The Way of Heaven” Mortarion is described as having changed: from a persecutor of psykers he turned into a seeker of otherworldly knowledge. Legion wars raged on the Eye of Terror with unknown force, the primarch was forced to find new weapons for the Plague Landing.
The ascension of Mortarion took place in Guy Haley’s novel “Grandfather's Gift”: he turned into a demon prince and received a xenos soul as a gift, the victory over which the Emperor stole on Barbarus.
Conclusion
In the track of the Russian group Keepers of Death “The Mortarion Saga”, it tells of the rise and fall of the primarch. The original of 2011 and the re-release of 2016 became the band's most popular tracks.
Mortarion at Warhammer 40,000 held the view that a person seeks freedom from any coercion and intimidation. After accepting Nurgle’s corruption, pride and arrogance turned against him: now the “Death Guard” spreads the plague so that the fall would not be shameful when the corruption would plague the Galaxy.
The moral of the saga is this: if the gods want to experience, they throw against the individual the strongest qualities of his personality. Primarch Mortarion did not pass the test, for he had betrayed himself.