Here Comes the King of the Underworld!

When he woke up, he was in a forest. It seemed that he had become a ghost. He had memories (or rather, knowledge) from before he died. And from there, the story of Shu Archlight began. With the power of absorbing life force, he steals magic and accumulates it to evolve. He analyzes magic and enhances his fighting strength. This is a story about the protagonist, who became known as the King of the Underworld, and his partner witch as they travel the world.