Since Games Workshop's Army of Your Dreams competition is closed for new entries, I thought I'd share what I entered. Remember that this is just 500 words.
The Empyreal Hunters were chosen from those who defied Chaos through cunning and wit, prosecuting campaigns of harassment and misdirection against the Enemy. Even when they were eventually caught, these heroes had the last word as Sigmar plucked them from the realms to become Stormcast Eternals. After the Scouring of Azyr, the Empyreal Hunters spent little time in Azyrheim. Instead they roamed Azyr in search of new places, beasts, and wonders to share with the Eternal City, becoming known as storytellers and pathfinders among the free peoples.
Now that the God-King has struck back, the lords of the stormhost decreed that their armor should shine like the stars, for they no longer needed to hide. As such, the Empyreal Hunters have replaced their original midnight blue with a brilliant gold, retaining the porcelain white masks and adding azure accents as a nod to their past. They still retain records of the stormhost's deeds in beautiful and intricate heraldry on their shoulder pauldrons, showing the enemy why they should fear Sigmar's chosen.
The Empyreal Hunters has more Harbinger chambers than any other kind, their Knights-Azyros and Prosecutors racing ahead to bring the earthbound members of the stormhost onto the battlefield with heavenly bolts after the Knights-Venator and Judicators thin the enemy ranks. Then the Paladin retinues smite the enemy commanders and war-engines while the Liberators crash into the hordes of darkness. The Hunters employ this strategy to throw the enemy into disarray and disrupt the battle lines, adapting the hunting tactics their members used in their mortal life to fight the Enemy.
Their brother stormhosts often admonish the Empyreal Hunters for lacking discipline in their ranks. Indeed, the majority of the Stormcast Eternals that make up the stormhost are headstrong and loose cannons, having been drawn from the wild people of the Realms. However, they share the same grand purpose as their brothers; the liberation of the Realms from the tyranny of Chaos. To temper their headstrong nature, the Hunters foster bonds between warriors and are led by those who can forge the disparate personalities into a formidable whole.
Foremost among the Empyreal Hunters is Lord-Celestant Orin Skyborne, who leads his Harbinger chamber on wings of celestial light and wielding a thunderous Lightning Hammer. Surprising both Orin and the God-King was the overwhelming support for the humble woodsman to be confirmed as a lord of the host. He accepted the position of Lord-Celestant with minimal formality.
In his mortal life, Orin was the cunning Arryn who evaded and harried Nurgle's plaguehosts in Ghyran for years before he was cornered. Even then he had the foresight to lay traps and pitfalls for his pursuers, killing and maiming many of Nurgle's blessed. It was a Lord of Plagues named Hurgk Blight-Tongue who caught Arryn, watching the mortal waste away from Nurgle's Rot with malicious glee when Sigmar plucked Arryn from his grasp. Only Arryn's laughter and Sigmar's thunder rang in Hurgk's ears as victory was lost to him.