The term 'unbeatable rivals' can be summarized as two people or groups who are in a constant competition with each other, and neither can gain an advantage over the other. The crossword solution for this term is 'NEMESES, ' which is a Greek word meaning 'an opponent or rival whom a person cannot best or overcome.' This term is often used to describe two people or groups who are in a constant battle with each other, and neither can gain an advantage over the other.