Final answer:
The question lacks context and cannot be accurately answered; clarification about the nature of the 'clans' and the context is needed to provide a relevant response.
Step-by-step explanation:
The question you have asked does not appear to correspond to any known educational subject or context. The Greek alphabet, which is mentioned in the provided information, does not include 'clans' but rather lists letters used in the Greek language. If you are referring to sororities, fraternities, or any other kind of groupings often named after Greek letters, they do not have universal 'rival' relationships as it might be implied in the question. Such rivalries, if they exist, would be specific to certain institutions or works of fiction. Without additional context, providing a direct answer is not possible.
The Greek alphabet provides a list of letters that represent different sounds. Lambda is one of these letters, and its rival is Delta. Gamma, Epsilon, and Theta are other letters in the Greek alphabet but they are not direct rivals to Lambda.