Complex sentence: It is unclear what exactly constitutes Dark magic in the wizarding world; however, it is the most common form of magic used by criminals, and dangerous spells used by others in the books are frequently labelled "Dark".
Split into simpler sentences: It is unclear what exactly constitutes Dark magic in the wizarding world. It also appears to be the most common form of magic used by criminals, and dangerous spells used by others in the books are frequently labelled "Dark".