In the surroundings of Eleusis there lived Cercyon, a huge wrestler and notorious robber. He was the son of Branchos (or Poseidon, or Hephaestus) and the Nymph Argiope.
Cercyon forced passers-by to wrestle with him and killed and robbed them one by one during the fight.
But when Theseus encountered the bandit, he had no fear, because he was strong and was known for his talent in wrestling. So he fought with the bandit and, banging his head to the ground, he killed him with ease.