Originally Posted by Malus Darkblade
I've read TEG like 5 times and I don't recall having read that.
So I guess you missed the line where Ravenor stated that as a teen he had no psychic abilities of his own. He was a mirror as in he was able to be a psyker as long as another psyker was in range.
He never gained any abilities that he could use alone till he became a grey knight. That was the emperor's gift.
Also, consider his inception into the Grey Knights. He would normally have been considered too old for recruitment. But his sheer potential for psychic power, even among Grey Knights, made them reconsider, and start the implantation process on him. So despite his age, his drug history, and his relations with Ravenor, who was in deep shit at the time, they still decided to give him a chance. And Grey Knights are only picked out of the most talented and powerful psykers the Inquisition finds. So the fact that they were willing to take that gamble on him, based purely on his potential psychic might, speaks a great deal about his inherent power. And then also consider the fact that some of the most powerful, skilled and talented Grey Knights (which, by them being Grey Knights, should easily put them amongst the most powerful psykers in the Imperium), recognised his powers, and considered him a potential future peer.
So, if you take all that into account, and then take into account his feat of shattering Angron's blade, Hyperion is easily amongst the most powerful psykers shown in the fluff, even if he doesn't have his own special Codex supplement.