In the ancient city of Eistenpolis, greatest metropolis of all the fractured, squabbling principalities of the now-dead realm of Arcadia, Apollyon plied his trades as a humble smith and part-time soldier. He lived, laughed and loved, earning a respected place in his community and raising a large, happy family. He was proud of his children, his neighbors, his people and his gods. When the trouble began, he was a voice of calm and an advocate for fidelity to Arcadia's pantheon and loyalty to Eistenpolis' citizens. He did not see the corruption slowly taking root among his friends and his neighbors, turning them against him.
When the hordes of Chaos reached Arcadia, the city-states allied and drew up a massive army resplendent in their bronze and leather, many thousands of spears strong. Apollyon stood in the front rank, closest to the enemy. Just before battle was joined, though, half the army turned on their brothers in arms, slaughtering them by surprise. The Battle of the Crimson Fields, as it would come to be called, was a complete and total rout, and Arcadia would be utterly destroyed save for the fallen. Its people, its culture, and even its tongue would be lost to the sands of time, existing only as whispers on the wind and a now-barren blight on the face of Ghyran.
Apollyon was one of the first to die. His soul was taken up by Sigmar, who reforged him into a Stormcast of the Anvils of the Heldenhammer to make war against his enemies. Still-grieving, Apollyon found what he thought was camaraderie and solace among the shared pain of the Anvils. He fought loyally and well in the titanic battles that marked Sigmar's return to the Mortal Realms, rejoicing in his ability to avenge himself on the enemies of the God-King.
This came to an abrupt conclusion during the campaigns against the Children of Sigmar. In one battle, Apollyon's entire Chamber was slaughtered- all save he, who managed to fight his way free from an encirclement with a hammer in each hand.
Recalled to Azyrheim, he found to his horror that his brethren did not await him. He was told the awful secrets of the Reclamation Engine, of the legions swayed to Nagash's service, armies that now counted his former comrades in their ranks. Twice-betrayed, Apollyon swore to never rest until he had made every one of the betrayers pay ten times over for their treachery. Painting his armor black, he donned a ferocious horned helmet to put fear into his foes' hearts when they saw him.
Now, he is Brother Steelios' left hand, the Ninth, the Betrayed, the Black Legionary, the Heart of Darkness. Where other Stormcast go into battle singing praises to Sigmar, his battle-cry is always "Smite! Kill! Burn!" He exists not for victory, it seems, but for battle. Countless reforgings brought on by his manic pursuit of vengeance have left him a shell of the man he once was- the father and the smith are now all but dead, and all that remains is the berserker. Fortunately for him, the Mortal Realms are vast, and enemies worthy of his mettle are always in supply...
The storytellers call Brother Apollyon the Black Legionary, the Heart of Darkness, the Ninth, the Twice-Betrayed, the Lash, the True, the Empty, the Father, the Smith, the Voice of Arcadia.
(The Children of Sigmar and the Reclamation Engines are not my creations. I took them from here: https://descentintochaos.wordpress.com/2016/04/19/children-of-sigmar/)