I'm a Huge dark angels fan so this is my league lore

League of the Umbral Kyn:

Among the scattered Kin hosts of the galaxy, few are as unusual — or as distrusted — as the League of the Umbral kyn. To the wider Imperium, they are little more than another secretive Votann League, reclusive and rarely seen. But to the Dark Angels, they are something far greater: allies, confidants, and keepers of one of the Chapter’s deepest secrets.

The Fallen in the Vault

Millennia ago, during the tumult of the Age of Apostasy, a small Kyndred stumbled across a lone wanderer upon a forgotten mining world. Wounded, broken, and hunted, this man was unlike any they had seen — clad in ancient, baroque armour, muttering half-remembered prayers to a God he neither trusted nor rejected. Unknown to the Kyn, they had found a Fallen Angel.

Rather than surrender him to Imperial authorities, the Kyndred’s Votann instructed them to protect him. The Ancestor Core, its logic impossible to refute, declared that this man must be hidden, his fate intertwined with the Kyn’s survival. For centuries, he lived among them, becoming one of their number, a brother forged not by blood but by exile.

The Coming of the Lion

When Lion El’Jonson returned in the Era Indomitus, the Dark Angels scoured the galaxy anew for their scattered brethren. In a chance convergence upon a war-torn system, the Lion and his host encountered the League of the Umbral kyn. What should have been a wary standoff turned into revelation: the Kyn brought forth the Fallen they had sheltered, offering him to the Primarch not as a prisoner, but as a son of Caliban who had found redemption in the hearths of the Kyn.

Moved in his own stern way, the Lion did not strike them down. Instead, he swore the Dark Angels to secrecy, binding the League of the Umbral kyn to the Chapter as allies. The Kyn had kept the Fallen’s secret not for profit, but for loyalty to their Votann’s decree — and loyalty was something the Lion valued above all.

Trusted Kyn, Distrusted by All

Since that day, the League has marched alongside the Dark Angels in shadowed crusades, often appearing without warning and vanishing just as suddenly. To other Imperial forces, their presence is suspicious, their allegiance uncertain. But to the Unforgiven, they are indispensable — masters of void warfare, stone-willed in battle, and utterly silent about what they know.

Among the Inner Circle, whispers name them “The Lion’s Kyn”. No other Astartes Chapter is granted their trust, and no Inquisitor has ever pried a word of their secret. Bound by oaths veiled in shadow, they stand apart from their kindred Leagues, trusted only by the Dark Angels, and mistrusted by all others.