Neither Segarus nor Caelum showed any signs of fatigue, but Segarus knew the gap between them even though they were at the same level. Segarus only has his magic to fight against him, while Caelum has his aura, but the sheer might of the aura Caelum wielded was beyond what anyone could comprehend. It was his because of skill with the sword was unparalleled; he had attained enlightenment through the sword, and he was one step away from becoming a deity.
Segarus knew that fact, and he wanted to kill Caelum before that happens.
In order to become a deity, one must pass through the tribulations. The type of tribulations one must go through depends on the individual's karma.