Rathrea

The rathrea were a type of undead dragon from Returned Abeir. Dragonborn called them ghost dragons.

Background
Rathrea were created when dragons failed to awaken after having consumed arambar. Those dragons eventually transformed into wraith-like, incorporeal glowing white spirits who had the ability to divine the future. Such ability was limited to certain areas of information and time-spans.

The rathrea didn't lived for much time, fading away in a few weeks. According to Gauwervyndhal, those rathrea "ascended" after they disappeared.

History
The Empress Dragon Gauwervyndhal prized the rathrea for their ability, as it allowed her to outmaneuver her enemies. Gauwervyndhal forced many of her dragon servants, even her own children, to consume arambar in order to procure more rathrea.