Jinlong

One of the seven Serpents of the Void Dragon Mag'ladroth, the "Golden Dragon" was imprisoned on ancient Terra. Code-locked into quiescent stasis by the Old Ones as were all the Serpents, Jinlong nevertheless slept uneasily. During the Dark Age of Technology its dreams influenced the humans living near it on the great Asian plateau, inspiring them to construct the machine that would later become the Golden Throne. Jinlong's prison-body itself was buried deep beneath the Himalazian mountains of Terra - and it may be postulated this was the reason for the establishment of the fortress of the ancient Sigillites and, later, the Imperial Palace within that mountain range. Located beneath miles and incalculable tons of rock, Jinlon's prison-body had been buried geological-eons ago, before the formation of the mountains themselves.
The Sigillites were dedicated to the preservation of Humanity's greatest achievements; the incomprehensible technology of Jinlong was thought to be a product of human brilliance during the Dark Age of Technology and so they resolved to protect it. Unable to move the massive and deeply-buried golden prison, they instead built their fortress atop it.
Long after the fall of the Sigillites the Imperial Palace was constructed from the ruins of their fortress, and their deepest and most well-guarded treasuries became the first Black Cells; oubliettes imprisoning entities or artifacts that could annihilate the Imperium of Man. As deep as the Dark Cells were, Jinlong was buried deeper still - reached only by vertiginous shafts cut through the mountains to the bedrock itself.
But in the intervening millennia the codelocks had become damaged, weakening the imprisonment Jinlong endured. Perhaps damage had been wrought by the tectonic upheavals which had birthed the mountains, perhaps Jinlong had broken the encryption, perhaps too-curious Sigillites had heeded the Golden Dragon's whispers and loosened its bonds, perhaps simple time had corrupted ancient data. For whatever reason, Jinlong's code-prison was no longer a monstrous golden monolith buried beneath the mountains. Although by no means free, Jinlong's prison-body was a vast labyrinth taking the form of a golden forest of blossoming trees, ever-changing as the Golden Dragon dreamed. Perversely, there were tended gardens and groves - bridges over babbling streams, gazebos overlooking placid ponds in which ornamental carp swam, all made of the golden crystal of the alien machine-intelligence. Through these gardens and the paths of the forest, running through the fallen blossoms, weird and grotesque golden creatures scampered. The Shadowkeepers of the Black Cells realized these were the unquiet dreams of Jinlong, fragments of its horrific intellect given freedom and form. Left unmolested and to their own devices, these "Dragonettes" would gnaw at the roots of the forest, further damaging the codelocks and eventually freeing their monstrous parent.
The Custodes were skilled warriors and fighters, but hunting wily creatures in in the ever-shifting landscape of the forest of golden blossoms they were at a disadvantage; no such landscape existed on Terra and had not for millennia. The Shadowkeepers hunted the Dragonettes but with no great success - until the Primarch of the First Legion was discovered and word of his youth on Caliban came to Terra.
The Lockwarden of the Shadowkeepers immediately realized the woods of the Lion's adoptive homeworld were akin to the forest of golden blossoms - complex, shifting, uncertain and dangerous and populated by creatures with the instinct of animals and a fraction of the intellect of a man. Petitioning the Emperor to recall His son to Terra, the Lockwarden formed an elite corps of Shadowkeepers who would patrol the golden forest and hunt the Dragonettes.
For a Terran year, Lion el Jonson prowled the forest of golden blossoms and trained the Shadowkeepers, who came to be known as the Hunting Lions in honor of the Primarch who had trained them.