Empyrean is a word derived from the medieval latin word "empyreum" and ultimately from Greek "empurios" which meant "The highest reaches of heaven", believed by the ancients to be a "realm of pure fire or light" and also "The abode of God and the angels; paradise".
The modern dictionary definition is:
1. [n] the apparent surface of the imaginary sphere on which celestial bodies appear to be projected
2 . [adj] of or relating to the sky or heavens; "the empyrean sphere"
It has also been used as an adjective by M.S.Dworkin meaning "well-meaning ineptitude that rises to empyreal absurdity" (which says it all!)
The Nottingham pagan group called The Empyrean was apparently formed so long ago that currently no one can remember the exact date! A lady called Amy Simes was a prime mover in the organisation but she no longer attends regularly, finding it a bit of a treck from the United States! The group name was also originally conceived as an acronym:
so now you know!