Shorter than Asch's temper!
When an Entelexia dies, as long as it lived long enough and absorbed enough aer, an apatheia appears. An apatheia is solidfied aer that the Entelexia had taken in through the course of it's life. Brave Vesperia found a way to turn apatheia into spirits. The apatheia is also known as the Clear Ciel Crystal.