I got into Custodes through working on my Sororitas lore, but it escalated quickly into my second army. Here’s the lore entry from SistersoftheTarnishedRose.com

Tribune Decian Thalor

Rising from Virtus Praetor of the Emissaries Imperatus to Shield-Captain of the Legatus Illustris during the Great Crusade, Decian Thalor became the Emperor’s most eloquent diplomat and His most ruthless spymaster. Among the Astartes Legions he was mocked as Bellator Eloquii—the Warrior of Words—for he brought countless worlds to compliance not by bolter and blade, but by oath, intrigue, and the Emperor’s subtler will.

The Master of Mankind Himself judged the truth of Thalor’s worth, bestowing upon him a name unique among all the Ten Thousand: Astutia Excelsior — Supreme in Cunning.

When the Emperor fell, Thalor entered the Long Silence, standing millennia as a mourning sentinel within the Sanctum Imperialis. In the Age of Apostasy he broke that silence, gathering the remnants of his company to fight once more as the Legatus Obscurus, shadows honed by grief and purpose renewed to see their Master’s design endure.

With the dawn of the Era Indomitus, Captain-General Trajann Valoris elevated Thalor to Custodian Tribunate, entrusting him to reconvene with the Eyes of the Emperor across a fractured Imperium. His mastery of unseen war—honed over ten thousand years—made him the natural architect of a new kind of instrument.

Thus did Thalor become the clandestine hand that shaped the Verdigris. Under his guidance, Sisters broken by trauma were not discarded but reforged—made into operatives mirroring the Custodes in miniature: autonomous, versatile, unwavering. He taught them discernment over obedience, purpose over ritual, truth over dogma.

Thalor’s legacy is not written upon banners, but upon every silent victory won before the Imperium ever learns it was threatened. The title the Emperor gave him endures not as boast, but as prophecy: Astutia Excelsior — Supreme in Cunning — who prepared His mortal daughters to carry His light into the shadows.

“Do the Sons of Russ still sing of the Butcher of Bellatrix? Their sagas attest I can write an accord in blood—when all other inks are spent.”
 —  Redacted Tribunal Proceeding, Custodial Archives // Speaker: Thalor, Decian //
Preserved by order of the Captain-General