I Swallowed The Heavenly Demon summary:

40 years ago, three mysterious seeds fell to Earth, unleashing countless monsters. Chosen beings arose to fight against these monsters. Those who manifested the powers of this world's heroic spirits into weapons or blessings were called heroes.Min Jae-ha, who aspired to become a hero, wandered the front lines but couldn't achieve his dream. Giving up hope of becoming a hero, he now works at That’s Real Company.One day, while investigating a mineral dungeon for work, Min Jae-ha steps on a mysterious summoning circle and unexpectedly gains an extraordinary heroic spirit, one he had always dreamt of contracting with.However..."How many times must I tell you? I am alive!""Aren't you technically neither dead nor alive?""So what can you even do inside my belly?"Why has this extraordinary heroic spirit taken residence in Min Jae-ha’s belly instead of manifesting as a weapon or blessing?"Look up to me, and I shall teach you martial arts."