This is just a prologue to see how this idea goes. Reviews are welcome, and I may or may not extend this story. Sorry if I get any context wrong; I'm not completely familiar with the whole 40k universe. Anyway, enjoy.
The Imperial Knights have an uncheckered past of heroism and bravery. The Imperium's finest pilots have gone down in history as some of the greatest heroes in humanity. But, deep inside the Imperial Archives is the hidden story of House Taggrin of the planet Geruva. Theirs is a strange story of a seemingly unique kind, which begins with the story of the Taggrin triplets.
The Taggrin House was readying to induct three new Nobles to pilot Knights. On this night they would sit in their Throne Mechanicums and become accustomed to them. As preparations began, events began to unfold in another system and within the Warp.
On the other side of the Imperium, Tzeentch's forces were on the verge of securing several artifacts sacred to the Dark Angels on the planet Jarroth. However, when they thought they would lay their clawed hands on them an ambush of Knights left barely a tenth of the force left. The rest scurried away only to be met by a force of Dark Angels, who finished of the demonic forces. Tzeentch was annoyed by the news and decided to make a show of strength to the Knights. He scanned the galaxy and found the Taggrin triplets were to be inducted shortly. A plan quickly fell into place, and Tzeentch set about making preparations.
As the triplets sat in their thrones, Tzeentch bound three daemons into the systems who quickly implanted the ideas of rebellion and self-service into the brothers' minds due to the fact that, unbeknownst to the rest of the house, the triplets had slight psychic potential within them. As the night wore on, the brothers soon found themselves all in a state of disloyalty to the Imperium, even with the conditioning induced in the rite of passage. When the night ended they made plans together and, as soon as they gained access to their machines together, fled the fortress, destroying the other Knights as they left. They were hunted by both Taggrin and other Imperial forces, but, as soon as they could, the brothers fled the planet with their machines. House Taggrin was shamed and soon fell to pieces, while the Triplets of Geruva still to this day are a bane to the Imperium and a hidden mark on the history of the Imperial Knights.
However, against Tzeentch's wishes, the brothers do not entirely serve chaos. While he had hoped they would turn to the Chaos Gods like Tainted Space Marines, the triplets in fact roam the galaxy with no cause nor an affiliation besides those that can pay the most or gift them with the best technology to outfit their suits with. At the current year the triplets reside on the jungle world of Ougetehre, readying for a job to raid a Guard Base. The three have not seen each other for four years after going into hiding after fighting off the combined forces of House Raven and Blood Angels to secure an experimental new Chainsword, called the Chainclaw, for themselves which they all now wield to great effect.
While the Imperium covers up the effects of the Triplets, they cannot avoid the fact that they do threaten them. Three Imperial Knights causing trouble across the galaxy is difficult to ignore at the best of times. There are times, though, that they grudgingly require their help, but these almost always end with the Imperium betraying the brothers and them escaping. The Tau and occasionally the Eldar require the aid of huge war-machines but keep any transaction secret for fear of any citizens believing that they're rulers rely on any humans. But all the same, the Triplets of Geruva are a force to be reckoned with and will remain free and proud till the day they all die.
