Amp how? Do you mean when Lily Fergus was using the magical energy from the magical box to amp himself. If so Fergus was just using that energy to draw out some part of his original existence.
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Grimacing in pain, Fergus explains further. If life and death truly are mixed together here, then it should be possible for him, the "Fergus Mac Roich on his way to adulthood, who does not exist", to be connected to the him that was supposed to be here, for an instant.
Da Vinci is impressed that he'd think to link "Fergus's death" and "Fergus's rebirth" together like that. She likens it to a voluntary pseudo-resummoning. Certainly, he would be able to come into contact with the concept of the "correct Fergus" for a second, but in that process, he would be damaging himself severely. How would he recover?
Fergus says it's alright: he has one of the treasure boxes from the undersea palace in hand. It is the same one Columbus had tried to use; having fallen from his hand, it had rolled towards Fergus's feet.
Fergus: There's no time. Let's go. I'll take in this pain and death--- and at that instant, use the magical energy in the box to regenerate myself. At the same time, in this gap, I'll try to drag out some part, any part of my original existence--- and make it mine!
Fergus shouts and holds out his blade.
Fergus: Stretch. Stretch, like a rainbow. It is infinite in length, yet still a sword. The rainbow sword of infinite length. This is--- the spiral sword! Therefore, the Spiral Rainbow Sword!
The short sword that Fergus once wielded now looks the same as the one his older counterpart uses. A spiral of rainbow light begins to form around him.
Fergus: Even with a wild swing brought about by anger, this sword can easily cleave apart three hills. In other words, it bears the meaning of "destroying the land"--- therefore, I will use this to destroy it altogether. The Pillar of the Demon God that is perplexed by eternal life and death. And beneath my feet, this floating earth that bears the name of fantasy!
All of the magical energy in the area is being sucked in by Fergus's rotating sword. And as it receives that energy, the sword begins to rotate even faster.
Fergus: Overflow, rainbow, beyond the spiral's horizon! ---CALEDFWLCH CALADBOLG (Ultimate Rainbow Sword)!
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Base on this we can conclude that the Caladbolg used here was just it original strength if used by his adult counter part. In fact in the passage it even say that Fergus Lily sword was initially different but then transformed into the one used by his older counter part.