Misato Kyoichiro, the leader of the strongest group of adventurers, the "Golden Seekers," was betrayed by his comrades and left to die. 3uv3r

"I won't just die like this!!"

In his final moments, he activated a reincarnation spell. Kyoichiro successfully reincarnated twenty years later as a student named Asai Kyosuke, a wielder of the "Void" attribute.

"Void" is an attribute mocked for being "attributeless," but Kyoichiro knew the truth of this attribute, thanks to the "Time" attribute he had in his previous life.

The "Void" attribute has the potential to sur the "Time" attribute, which was seen as the strongest in his lifetime. Thus, reborn as Asai Kyosuke, he returns to the Adventurer Academy, determined to purge all evil from society; all to get revenge on his former comrades, who betrayed him out of greed.

And...

"Asai Kyosuke, I won't let anyone look down on you ever again."

...to reach the peak of strength, something he could not achieve in his previous life.