I Only Level Up When I Seal My Enemies

After entering Astrionis, the game world that had merged with reality decades ago, Riven awakened a class… not an ordinary one but a mythical class. The highest rank a class could reach. At first, the power it granted seemed very ordinary and had more drawbacks than benefits. It gave him the ability to seal living beings, an ability that didn't seem too powerful to him, and what made the situation even worse was that he couldn't level up by killing monsters to gain exp. Unsure of what to do, he tested his power by sealing his first enemy and realized that he only levels up after sealing his enemies. Not only that, every enemy he seals becomes his subject becoming a step towards an unimaginable army. But the smaller his army becomes, the weaker he becomes because losing his subjects destroys the very foundation of his powers. — Description from novelcodex.org