A brief description of the manga The Executed Sage Is Reincarnated as a Lich and Starts an All-Out War:
Sage Dwight, together with Heroine, defeated the demon king who plunged the world into darkness.
However, despite all their merits, they are accused of treachery and want to be executed by throwing them into the valley of the dead. But a wise man does not want to just give up.
He becomes undead and his main goal is to take revenge on the abusers and change this rotten world.