During the closing years of the Unification Wars, the proto-legions began to take shape, forming into the forces they would one day be known as across the galaxy. All of these forces were secured and kept secret from the rest of the Emperor’s own forces, save his innermost guard, to safeguard them against hostile action and from potential espionage. As such, the origins and early deployment of several early Legion gene-strains were occluded beyond the modern record's sight, making their origins hard, but not impossible, to pinpoint. There are three however who go beyond mere secrecy into the realms of full and deliberate obscurity, held apart from the other seventeen by an edict directly from the Emperor, his agents ensuring that their veiled creation would remain unspoken of openly, even though it was known across the other Astartes forces.
These three legions, soon referred to as the Trefoil, were numbered IVth (Star Guardians), XIIIth (Consuls Exemplar) and XXth (Hound of Surmah) , their creation taking far longer, or else purposely delayed by the Emperor himself for unknown reasons. This element of mystery surrounding them can be seen to establish a distance between the three and the others, at least subconsciously in the minds of their fellow Astartes, particularly in regards to their earliest intake of Initiates, around which dark rumours circled, and which would later blossom into a level of distrust. Some the legions would overcome this distrust in time, the XIIIth soon becoming a paragon of the Imperium, while the IVth and XXth would struggle to overcome this initial suspicion, this compounding other issues and causing them to often be distrusted or outright disliked by the other legions.
These three legions would be finished after all their peers, and in the case of the IVth and XIIIth would only see action right at the end of the Unification Wars, the XIIIth working to overcome their dark origins reputation as they marched across Terra, while the IVth would make their own debut at the Pacification of Luna, securing their reputation in the void, before descending back to Terra for the devastating Assault on the Tempest Galleries. Only the last of the Trefoil, the XXth, would remain fully veiled during the Unification Wars, only seen by those outside the Trefoil in those early days at the final unification of the homeworld, a trio of Astartes standing apart at the ceremony, only to vanish upon its completion, assigned to their own compliance actions until finally they fought alongside their cousins during the First Rangdan Xenocide, their dark reputation forever solidified in the minds of the others.
Eventually, the Trefoil would be all but forgotten by those outside its ranks, few remembering where the initial reputation stemmed from, instead focusing on the legions they had become and drawing their own reputations from there. Those that were part of it did remember however, never forgetting, but always unsure on what purpose they had initially been designed to undertake.