Why did Nie Feng become a demon?

In order to defeat the godless, Nie Feng and Bu Jingyun need to learn from Mundus's Excalibur. After selecting the magic queen, they thought that Nie Feng could control magic, so they asked Nie Feng to learn the magic sword. Although, he finally defeated the godless. However, because of the fighting, Nie Feng's magic can't be controlled, and Long Mai must be taken away to suppress it. Otherwise, the magic will end, but then Long Mai is destroyed by the tyrant and Nie Feng is in danger. . . That's it.