The term 'most mysterious' can be described by three crossword solutions: Eeriest (7 letters), DEEPEST (7 letters), and Shadowiest (10 letters). Eeriest is an adjective that describes something as eerie, or strange and frightening. DEEPEST is an adjective that describes something as being the DEEPEST, or having the greatest depth. Shadowiest is an adjective that describes something as being the darkest or most obscure. Together, these three words describe something as being the most mysterious, or the strangest, DEEPEST, and darkest.