Fallen Angels are beings that were once associated with God, but were cast out for deceiving, betraying, or going against His will. From the time of Genesis in the Bible, the sons of God (the Angels that once lived in Heaven) went to Earth to marry and mate with the beautiful daughters of the human race, thereby giving birth to the Nephilim. Becuase of this, God saw the Earth corrupted and sought to destroy it. This was the action that caused the many Angels to fall from God's grace, and are now left to wander the earth, or in the residence of Hell itself.
Eventually, the Angels that were cast out were then known as the Fallen Angels, and created their own cult known as The Fallen. When time passed, the Fallen Angels had associated themselves with Demons and Devils, therefore blending the species of both heavenly and demonic attributes and powers. The clans in the far future of these mixed bloods would be come to known as the Glacio Clan, the Venrinarde Clan, the Ralko Clan and many more.