Radiant Idols are angels from Syrania that have been banished, and condemned to spend their eternity on Eberron. They aren't all evil, and they don't hold a candle to the evil of fiends, but they aren't the bastions of good that people expect angels to be. A radiant idol could have fallen for any sin, but by far the most common amongst them is the desire to be worshipped by mortals.
Radiant Idols are cursed to never again feel the sky, and they are unable to fly. As immortal spirits, their physical forms often reflect this, though the form that this curse takes on them varies between angels. Some will have bloody stumps that never seem to heal in place of their wings, whilst others retain their wings but they are unusable, whether due to bindings or because the feathers have all fallen away. So powerful is the curse that prevents a Radiant Idol from flying that nothing can fly in close proximity to them.
Radiant Idols generally attempt to establish cults around them once they make it to the Material Plane, setting themselves up as a god. The "domain" of their god persona is usually related to the domain they embodied when they were an angel of Syrania, and the activity of the cults they establish will be related to this concept.
The Syrania manifest zone that encompasses Sharn has lead to the fall of more angels than any other zone, the proximity of so many mortals proving too tempting to ignore for some angels. As such, Sharn has the greatest population of Radiant Idols on Eberron, but even so there are only about 5 or 6 of them at any given time.