Sinis was a bandit on the path to Athens by land, 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 to meet his stepfather, he runs into Sinis, who tries to fling Theseus out of a tree by telling him to try hold down the pine tree. However, Theseus tricks Sinis into tying himself to two trees and, with the help of Sinis’ daughter Perigune who hated him, flings the bandit to his own death.