A Dragonslayer's Peerless Regression

At the moment of his death, mysterious words appeared before his eyes:
[The soul of an Elder Dragon is engraved upon the will of the one who persists.]
[An irreversible, promised power is being activated.]
When he regained consciousness, he found himself back in the Dragon Slayer family that had abandoned him, 20 years in the past.
“If I can become strong, I’ll make use of anything and everything.”
Armed with a newfound, immortal power, the legend of a Dragon Slayer in a class of his own begins.