Cercyon was the king of Eleusis. He was well known for his cruelty to his daughter, who he buried alive when he realized that she had a child with his father, Poseidon. After that, she was transformed into a stream, Alope. He was also known to wrestle people for the throne, but since he was better, he was able to kill them. He was killed by Theseus.
In Percy Jackson's Greek Heroes, He is described as an insane man who terrified the locals and destroyed temples, rampaging around in gold spandex pants. Apparently, he was "as clumsy as a sow." Thus, he was killed easily by Theseus. After his death, Theseus refused the throne and asked the town who was the smartest person in town, and they hesitantly pointed to an old man, most probably Hippothoon, his grandson.