Fallen Angels are divine servants who became corrupted and were exiled from heaven. There are many reasons for their descention, some joined the forces of evil during the Ogre Battle, some were corrupted by outsiders, some declared war on humanity, some even willingly turned their backs on heaven or choose a fell master...
In terms of appearance, they typically have black wings and armor, purple robes, dark hair and red eyes; some mutate into fully or partially demonic appearances, with bat wings and horns. Since they are spiritual beings, they can be bound to certain planes of existance or deprived of a physical form, thus depending on a human host to exist elsewhere.
Fallen angels are heavily featured in Tactics Ogre: The Knight of Lodis but rarely mentioned in any other game in the series. The fallen Shaher and his servants Lethe and Cirvante are the most notorious members of this race.
The Dark Angel is a generic variant of this race, as an enemy-only unit, unavailable to the player.
- While it's entirely possible to turn an Angel Knight into the Bane element, this won't turn them into fallens.