Which word indicates something that is beyond description and filled with deep mystery?

The word that indicates something beyond description and filled with deep mystery is "ineffable." This term is often used to describe experiences, feelings, or entities that are too great to be expressed in words or are so profound that language fails to capture their essence. It evokes a sense of wonder and an acknowledgment of the limitations of human expression.

While "extravagant" relates to being excessive or lavish and does not convey the feeling of indescribable depth or mystery, "inscrutable" refers more to something that is difficult to understand or interpret but may not have the connotation of transcendence. "Indecipherable" implies something that cannot be decoded or understood, typically associated with texts or codes rather than experiences or emotions that are ineffable. Thus, "ineffable" perfectly encapsulates the idea of something that eludes full description or understanding due to its profound nature.