It is the hypothetical source of/evidence for its existence is provided by: Old Persian atar "fire;" Latin ater "black" ("blackened by fire"), atrox "frightful" ("of fiery or threatening appearance").
"empyreal," mid-14c. (as empyre), probably via Medieval Latin empyreus, from Greek empyros "fiery," from assimilated form of en (see en- (2)) + pyr "fire" (from PIE root *paewr- "fire"). As an adjective in English from early 15c. The etymological sense is "formed of pure fire or light." In ancient Greek cosmology, the highest heaven, the sphere of pure fire; later baptized with a Christian sense of "abode of God and the angels."