Updated story as the one above was a first draft.
Primarch: XERXES - He was arrogant, manipulative, controlling, cruel, aloof, charismatic when he needed to be and absolutely evil to the core. He saw most humans as beneath him, something to use to further his position. He was not controlled by chaos nor did he worship it. He knew of its existence however. He saw those that worshiped the chaos gods as slaves and fools.
He hid his true feelings and motivations from his father and his brothers. He did not really get on with any of them but did the minimum required not to raise too many eyebrows. He was extremely powerful, both as a psyker (which he hid) and a warrior. He had a ferocious temper and anger which he always had under complete control unless he choose otherwise. Ultimately he wished to rule the galaxy as his plaything and would do whatever it took to realise this goal.
He also consumed an elixir, like the civilisation that brought him up on Constan. Its origins are not clear, but its production involved the misery and death of many humans. Of course while he still fought for the Imperium he kept this behaviour secret from all but his most trusted.
Before he was discovered the Legion fought with the Emperor in the unification of Earth and many years of the Great Crusade. They were always ruthless, uncaring, cruel and unforgiving but extremely efficient, methodical and effective.
They had a penchant for high tech weapons and had access to unique Standard Template Construct machines due to a forge world that was located in the same system as their home planet.
Xerxes was discovered third of his brothers and given command of the Legion. He quickly exerted his influence and filled the ranks with Space Marines from Constan who were absolutely loyal to him and shared his beliefs. They also consumed the elixir. This was initially hidden from the terran members of the legion but eventually all but one were brought on board.
One amongst their number, a Captain was apart from all others. He was called 'the one' a hero to many he saved from the Legion.
This Captain raised the alarm. The Emperor accompanied by Russ and his Legion went to go and deal with Xerxes.
Large battle occurred, what happened is unclear but Xerxes and a large portion of the Lion Knights manage to escape. As he leaves Xerxes tells his father that he will return one day and enslave galaxy.
Leaves, never seen again. Captain follows to save those he can.