A Wolf by the Ears (Book 1 of Forest Guardian)

Centuries ago, a great spirit of darkness tried to enter the Cardinal World, inciting a clash that tore the land apart and set it on the course of a new age. To dispel this darkness, the most powerful beings in the world combined their energy to seal it away, rendering themselves dormant in the process until they could finally recover the strength to rise again. Now an arrogant king has found the place where Rimuru is sleeping and devised a way to wake him up. Kept under control in a weakened state, Rimuru is plotting his escape... and his revenge.