Krulug is the god of the earth, dwelling within the earth's crust and rarely coming to the surface. His skin is as hard as rock because it is literally made of rock (a weak punch would do nothing to him).
His design is inspired by a beetle-shaped Digimon, as the artist has loved Digimon since he was a child and saw an opportunity to incorporate it in some way.
He was the first boss we designed for the game and was also an enemy we wanted to introduce in the first game but never did, so now is his time!