As Jack finished the third Undead Dragon, he turned to Duorda and began his walk back to the main fight. But the earth shook again as four more Undead Dragons revealed themselves.
Duorda commented, "You shattered their bones… A good move. That keeps me from resummoning them."
Jack challenged the dragon's charge, understanding that Duorda likely had more summons than the hero could imagine. If Duorda was old enough to know of Daruun and his true role in the world, then there's no telling how many different beasts or people had come to challenge the crypt lord over time. And remembering Duorda's earlier comment in the battle, Jack assumed that these Undead Dragons were once Duorda's challengers. This was more clear as the newest four Undead Dragons were different from the first three.