Zeliel is called the fallen because like many of his heritage, he was on a path to be a priest of the light.  But he was never one for the cloistered lifestyle and his chaotic inquisitive soul longed for a different sort of power from the healing energies of his fellow priests.  Instead, he found himself drawing on the celestial blood inside him to manifest a different sort of power, not the power of faith, but the arcane might of the celestial warriors.  He was soon easily discovered as having forsaken the more humble life of prayer to gain divine power in order to gain the more potent magics offered through arcane means.  He was branded Fallen and sent away.

Now Zeliel travels the realms seeking to show that he still serves the light, though in a different way.  He is a bit off put by the overly devout, often making sure to emphasize the fallen moniker to raise their hackles, but in the end he knows that all who strive for right are one, even though they may be estranged from one another.