No system + invincible + feel-good story + no female lead + fantasy

Chen Guanlou obtained immortality and secured a stable job as a prison guard in the imperial dungeon, beginning his daily routine of slacking off at work.

Over the long years, he watched men build tall buildings, watched those buildings collapse.

He watched them rise to the imperial court today and become prisoners tomorrow.

He watched their families flourish in wealth and power, watched them executed in three generations.

He watched royal power and wealth, watched nations fall and families destroyed.

What changed were the years and human lives; what remained unchanged was the ageless span of immortality.

Chen Guanlou outlived masters, outlived grand masters, outlived one great after another, until he finally became an invincible being.