The Mage of the Abyss: Imprisoned as a Rebel, I Gained an Immortal Body and Ultimate Power—Now I Rise as an Adventurer (WN)

Vim Abel is a court mage serving the Cyrus Empire.
However, the Empire, fearing Vim's overwhelming magic, imprisons him.

The labyrinth in which he is imprisoned is protected by a barrier that makes escape impossible, making it a top-level labyrinth.
Even in such a labyrinth, Vim manages to conquer the deepest part just to pass the time.

As a result, Vim's body unexpectedly gains the ultimate power of immortality.
One year later, Vim discovers a place where the barrier has weakened and escapes the labyrinth. He then heads to the neighboring Kingdom of Leoria, gains the favor of the princess, and rises to become an S-rank adventurer.

From there, Vim resolves to reform this world.