The three had come closer together to be able to see the skill that they intended. They all had learned to use light mana. With the right amount of training, Su had also learned to infuse light mana in to this unique attack which she had learned more and more about since she became a half dragon fully.
Walker was the only real outlier, but since he knew the way that manas flowed, knew the ways that he could make it move properly within his own body, it was easier for him to use. There was also the fact that he had long learned through his bond with Midnight how to use dragon fire even without having to use a dragon breath attack.
In a burst of heat and light, the three released the manas that they had gathered. Walker used a little extra mana to gather the three streams of white purifying dragon flames in to a singular potent purifying dragon flame that encompassed the entirety of the undead seal left behind by the immortal king.