Um. The current embodiment of the Emperor's Champion is not available to Imperial Fist successors. It is available only to the Black Templars. Black is their armor, to match Sigismund's own black armor when he was sworn, by Rogal Dorn's authority to be the Emperor's personal champion and bodyguard. His failure to save the Emperor from death drove him to take up a crusade against the enemies of Man.
The Emperor's champion is exclusive to the Black Templars as far as the rules go, because it was Sigismund himself who founded the Black Templars, and gave the shards of his black sword to be re-smelted and re-incorporated into a limited supply of black swords which are jealously guarded by the Black Templars. The hilt of his sword was reforged into the sword of the High Marshals which currently is held by High Marshal Helbrecht.
No other chapter has easy access to the Black Sword. While others may field what was once mistaken to be Emperor's Champions, they are now revealed to be company champions, who merely weild a powersword and a combat shield. These champions are still fierce foes to be met on the battlefield, but only the an Initiate drawn from the ranks of the Black Templars may truly wear the mark of Emperor's Champion.
PS: I looked through the Black Templar codex. I didn't catch anything that allows non-Black Templars to field the chump. I also don't believe the current embodiment of Codex Space Marines allows for it either. And Codex Armageddon has been discontinued as a source for Black Templar material. |