Rage

"I agree. I don't believe that in the history of the Ancient Battlefield, there was no one capable of such a feat. In fact, I believe there are two possibilities… 

"The first is that we are wrong from the very beginning and that there have been individuals who have succeeded."

"You mean they've been purposely erased from everyone's memories?" 

"Mm." Dyon nodded. "It's not impossible. The Unicorn corpse puppet I gained from Orcus was forgotten by me many times despite my having The Seal. It wasn't until recently that that forgetfulness no longer affected me. But, by then, the corpse puppet itself was already useless."

It was quite an unfortunate series of events. 

Dyon continuously forgot about having the corpse puppet by his side. And, by the time his soul was strong enough such that The Seal improved to the point of this forgetfulness fading, the corpse puppet was already of no use to him.