The combat strength of a controlled undead army was worlds apart from that of an uncontrolled undead army.
These skeleton soldiers and ghosts that drilled out of the ground had always been fighting separately. They were only instinctively attacking Roy and the others out of envy and hatred for the living. Such attacks could be said to be chaotic and posed no threat.
On the contrary, when Cassandra appeared, the undead army she controlled displayed great combat power.
