He flinched.
Wasn't the answer obvious?
What could cure the infection of cursed knowledge?
…Oblivion could.
In order to be cured, he had to forget the cursed knowledge before returning to the past.
'The Key of the Estuary.'
A mysterious Memory that had been there, in his soul, when he found himself in the mist on the first day of the Nightmare. A Memory that possessed a passive enchantment that did not seem to have any effect on him.
An enchantment that consisted of two parts — one directed outward, one directed inward.
And which had something to do with the Sin of Solace.
Sunny took a deep breath as the truth of the Estuary Key finally revealed itself.
'It's... a muzzle.'
The purpose of the mysterious Memory… was to prevent the Sin of Solace from sharing any knowledge of the previous cycles with Sunny. So that he never remembered what had happened in the past, and therefore never learned the cursed knowledge of Defilement.
That was one part of its enchantment. The other, much more burdensome part… was meant to make the Estuary Key immune to the change of cycles. To make it remain in Sunny's soul even if he crossed the boundary of the Source and was sent into the past.
That was why the Mad Prince had been so interested in the Great Tyrant slain by the Serpent King in Aletheia's Tower. Because the Tyrant remained dead no matter how many revolutions passed… its death had been made permanent, carrying over from one cycle of the loop to the next.
Back then, the Defiled madman must have already been trying to create the Estuary Key.
To cure himself of the Defilement… and make another attempt to conquer the Nightmare. No, not just conquer it, but do it in a specific way.
To erase the mistake he had made... or maybe the crime he had committed.
'Murderer...'
Sunny gulped for air.
At that moment, he realized something else.
There were only three of the Six Plagues left: Soul Stealer, Torment, and the Dread Lord.
The Mad Prince… was gone.
He did not exist in the Tomb of Ariel anymore.
Because he had crossed the boundary of the Source and sent himself into the past.
He had become Sunny.