The Hayabusa were once normal people who struggled to survive before they suffered from the hands of the Jubi. They were saved from the Jubi by the God they worship. The god is called Watatsumi Sanjin (Great Spirits of the Dragon, the Three Watatsumi Dragons).
The dragons came to the Hayabusa's by granting them some of their power and knowledge. Combining the power with Ninshu birthed their ability.
Wuji is a state where you can transit from existence to non-existence. Divided into Wujin and Taijin.
Wujin removes from existence while Taijin brings to existence.