Mordred

Mordred was a small black tom. He was the leader of DarkClan, succeeding the founder, Dark. He was eventually picked by his brother to be leader by his side, before stepping down with him to allow DarkClan to lead itself. Eventually, his niece, Eloen, became the next leader.

Trivia

 * Interestingly, in the Legend of Arthur, Mordred is Arthur's son, not his brother.