Kinki no Tensei Daimadoushi

One day, dungeons suddenly appeared. Humans with awakened skills set out to explore these dungeons in search of treasure. Our protagonist, Saeki, having no skills, was considered useless. In order to earn enough money for the treatment of his little sister with an incurable disease, he decided to become an adventurer anyway. But Saeki had a secret. In his previous life he was the most powerful mage in the world. Since his reincarnation, he had to level up to regain his former powers. This is the story of an incompetent who will turn the world upside down with his growing power!

(Source: Hyke Comic, translated)