Sinis was a bandit and Greek demigod son of Poseidon who killed his victims by flinging them from trees. He was killed by Theseus.
Percy Jackson's Greek Heroes
When Theseus is on his way to Athens, he runs into Sinis, who tries to fling Theseus out of a tree. However Theseus tricks Sinis into tying himself to a tree and, with the help of Sinis’ daughter Perigune, flings the bandit to his death.