There is no information on the origin of the Ascians, beyond that they are Zodiark's servants devoted to His resurrection that I'm aware of.

However, it is known that beings can be elevated to an existence similar to the Ascians. The Echo, supposedly, is key to this. The Sahagin Elder, the Lesser Ascians, and the Warriors of Darkness are all beings whose existence became similar to the Ascians - disembodied aetherial spirits. Exactly how they were elevated to this form of existence is unknown, but it does require physical death.