Rise From The Bottom summary:

The most powerful Blade King in the empire, Laycil's grandfather was executed for treason by the empire and the rest of his family was wiped out. From that moment on, Laycil's life changed drastically. Laycil was enslaved, humiliated, and subjected to human experiments. These experiments included being implanted with dark demon cells. Laycil was completely trampled on as someone at the bottom of this world. However, during a conflict with a bully, he awakened the power of the dark demon, Bloodline Remembrance. He thus inherited all the knowledge, experience, and skills of his deceased relatives, and was then able to start down the path of vengeance against the empire!