The term 'Unconquerable Rival' can be summarized by the crossword solution 'NEMESIS', which is a seven-letter word. In Greek mythology, NEMESIS was the goddess of divine retribution and vengeance, who would punish those who committed hubris or arrogance. In modern usage, the term 'NEMESIS' has come to refer to an unbeatable rival or enemy. Therefore, the term 'Unconquerable Rival' can be accurately summarized with the word 'NEMESIS'.