The Lich King's fourth skill is also a standard ability, Bone Arrow. A typical attack skill that can be fired singly as a regular attack or in a volley.
Du You didn't know if it was because the ones who died under his Dark Flame Arrow Array formed this kind of skill, because both skills were indeed very similar. This similarity referred to the surface appearance, not the actual magic effects.
Bone Arrow only possessed physical attack and the attached death Qi attack, and didn't come with the Soul Devouring Demon Flame like his own attacks.
After experimenting a bit, Du You discovered that the attack power was quite decent, not at all dishonoring the title of his Summoned Beast. With this addition, he had indeed acquired a formidable pure Magic-class Summoned Beast among his forces.
The sixth skill, Corpse Explosion, is relatively common among Undead Creatures but is not a weak one.
