The Dragon's Seal is broken and they join your cause to destroy the God of White Magic's plan to remake the Primordial God with himself as it's consciousness.

Update: Added siblings

(LONG BACKSTORY)

Before the universe was born, there were two beings, the Primordial Goddess; Hylia and the Primordial Dragon; Nyx. The two lived in harmony as they divided their power to new beings. Hylia became the Masters which further separated their powers into gods while Nyx kept one generation of seven dragons. After thousands of years of harmony, tensions began to arise when the dragons disapproved of the creation of humans due to considering them a threat to the natural order so at the gods creating them anyway, the dragons attacked the humans and the gods. They failed and were sealed in the northernmost continent in a location known as the Dragon's Seal. And out of human precautions of the dragons ever breaking the seal, the humans formed the first megacity of the human population, Dreigenheim far away from the Dragon's Seal. But in doing so, this drove the God of White Magic insane since his power was beginning to be forgotten which would make him powerless so he decided to think back to Hylia and got an idea to reform her so he did, killing the gods one by one and tricking others to reform the Masters and absorbing their power so he became unstoppable. This is where you come in. You knew that the only ones that could fight the God of White Magic now was the dragons so you broke the seal and freed the dragons. You explained your goal and they joined in exchange for their freedom.