This is the life of one of the Thousand Sons. He has no name, no more than he has a body. He has nothing. In nearly all aspects, he is nothing. He does as he is bidden, fights where he is led, follows his orders, finding nothing of value or satisfaction in doing so, nothing to give his existence worth. Years turn into centuries, as he moves through them like a machine; life, barely worthy of the name.
Then, in the most unlikely of places, he finds something that surprises him. In the charnel ruins of a world of the Imperium, a world that has fallen under the scourge of Chaos, he finds a soul like his own; a young girl, wounded, near death, having lost everything she held dear.
He feels something new. He feels pity. From some inexplicable place in the depths of his mind, he feels the urge to protect her. He is strong, this Son; strong enough to guard her against the darkness, strong enough to save her life when hope seemed lost. He cares for her as she recovers from her injuries, and despite his inability to communicate, she slowly realizes that he is genuine, that means her no harm, that there is no underlying fiendish plot. Gradually, the caring that he feels for her is returned.
And so, together they build an island of joy in a sea of horror. They find happiness, love, and it seems as if they have escaped from the cares of the galaxy. For a time. For there is one flaw in their love; the Son is ageless. The human is not. The girl grows to a woman.
The woman grows old.
Finally, after decades together, time claims what cannot be denied. She dies, peacefully, her last words "I love you so." Her soul rejoins the Warp, after a life better than many in this dark age. And the Son must go on, alone, with only the memory of his time of joy, tainted now by the agony of losing that which he loved. The Son must go on, forever.
In his mind's eye he sees the future set out for him; a barren path stretching into the ages. With the passive resolve of an automaton, he rebels.
He makes no declaration, does nothing to announce his silent revolt. The fingers of his gauntlets feel for the seals of his armor, fused shut for ten thousand years. They won't open. His armor has become a coffin, what remains of the man buried alive to fight and obey for all time.
And yet...he remembers the time he crushed the armored head of a loyalist marine on Badab VI. His fingers constrict around his neck, buckling and snapping the airtight seals, the might that made him invaluable as a slave now setting him free.
The Thousand Son removes his helmet and stares up at the stars with his own eyes for the first time in ten millennia. Only his eyes have long since turned to dust. Without the lenses of his helmet to see the world through, he sees the Warp, the winds of Chaos, and on them a delicate, luminous wisp of soul, its warmth and decades of quiet contentment so unpalatable to the daemons of the gods of Chaos that the fierce predators of the Warp flow around it like oil and water.
The armor clatters to the ground, and with an inaudible sigh, the dust within is carried away on the winds.