Su Yu slowly retracted the Netherworld Skull, his complexion becoming somewhat wilted. Utilizing the power of will to manipulate the skull took a great toll on his mind. Moreover, this power of will could only stimulate the divine power within the Netherworld Skull, but he had to throw it himself. His current speed of throwing the skull was too slow. If he encountered opponents stronger than the redhead man and his group, and if they were prepared, the Netherworld Skull wouldn't be able to hit them at all.
That's why it served as his secret weapon, to be used only in special circumstances when the enemy was caught off guard, to even have a slight chance of success. Therefore, just now he hadn't planned on using the skull to attack the few powerhouses of the Phoenix Clan. Instead, he used it to destroy the cliffs and release this long-haired woman who had been imprisoned for who knew how many years.
