Sanguinius knew pain.

For countless years, the Great Angel of Mankind had fought, tooth and nail for the prosperity of his race. From across the endless galaxy, fighting upon the surfaces of a hundred thousand different worlds - he waged war against the many foes who dared to oppose his Father's Imperium and their vision of a right, just future.

He had battled, slain and emerged triumphant against nearly every form of life which had stood in the way of humanity. From the most brutish of Orks, to the most enigmatic of xenos, to the most bloodthirsty of traitors - Sanguinius killed them all without exception, all for a great future he knew he would never see.

Sanguinius knew death. He saw it almost daily. Countless, nameless faces he saw when he closed his eyes at night - of a great and many number of men, women, and children who were thrown to the unrelenting fires of war, to serve as fuel to the ever churning pyre of hate. To suffer and burn amongst the endless wars waging across the galaxy. The people he had failed to save.

Sanguinius knew death. Intimately so. He saw it every time he and his Legion made planet fall. Every time the skies of another world would choke and clutter with the smog of bolter fire and plasma batteries - as the he, who was considered best of his brother Primarchs, soared across war torn landscapes with his angelic wings - The Great Angel would witness untold, needless death.

Sanguinius would mourn, for every time he made planet fall to wage war on behalf of a thankful humanity, his very own sons would be the hammer at his back, the shield to his front and the blade by his side. And for every battle fought, for every victory claimed - the soil beneath his feet would be stained crimson with the blood of Angels. Red as the color they wore proudly upon their armored pauldrons - the blood of his sons.

Sanguinius would fall into anguish upon every death - for his Legion, though courageous and strong, was unknowingly cursed. His sons, his Astartes, were doomed to their very core. Doomed to their flesh and bone, to their very genes. The genes that flowed from Sanguinius' very own body. Damnation came, not from the vast and ruthless enemies of man, but from within their very bodies. The damnation of his sons was his own fault, his own shame.

The Red Thirst, Sanguinius would call it.

Each Legionnaire of the IX that fell to the Red Thirst, Sanguinius would personally shed a single tear for, a tear of sorrow and regret, as a father was forced to euthanize his son. Mercy. It was the only way to keep his shameful secret safe. To keep the rest of his sons safe from the peering eyes of the Imperium. From his Father's cruel and calculated stare and unforgiving retribution. To save his sons, Sanguinius would have to murder a vast number of them.

For every Angel lost, Sanguinius would mourn, and he would be shamed - filled with anguish and regret. For the blame rested solely with the Great Angel, for it was his defected and flawed genes that damned his own sons to oblivion. It was his blade that stemmed the life of his fallen Angels. It was his weight that bore down upon their cursed souls.

If there was one thing in the galaxy that Sanguinius knew, it was loss.

But Pain - Sanguinius thought he knew pain. What could hurt worse than having to suffer the death of your own sons by your very hands? What cruel architect of fate could bring more suffering than such a guilty mercy? What could be worse than the cloak of endless war that raised a shadow over the rest of the galaxy?

Then, he, who he trusted and loved most dearly within the galaxy, fell to the darkness.

Sanguinius thought he knew pain, but nothing - not his countless battlefield scars, nor his Legion untold casualties - nothing could compare to the pain he felt when Horus fell to Chaos. When his favorite and closest of his brothers betrayed their Father. Betrayed the Imperium. Betrayed Sanguinius himself.

And then, in the final hours of the Heresy - his very own brother, Horus Lupercal, slew him. His own brother sent his great and daemonic power maul - Worldbreaker - to his chest. Crumpling his armour. Crushing his heart.

Thus, only when Sanguinius finally died, did he truly know pain.