The adjective empyreal isn't used very often, aside from poetic and religious writing. You might come across it in a poem, describing the summer sky in flowery terms, or in a more devout context to mean "heavenly." If you read the phrase "empyreal realm," it either means a divine paradise or a beautiful, awe-inspiring sky. The word empyrean is also correct, and both forms of the word come from the Greek root empyros, or "fiery."