The Ketos Troias or Trojan Sea Monster was a gigantic sea monster sent by Poseidon to plague Troy as a punishment for when King Laomedon refused to pay for Poseidon building the walls of Troy. When the Argonauts got blown off course they came across the monster and Herakles fought the beast. There are two beliefs of how he killed it. He either shot arrows at it and caused it to fall dead, or he used two overly large fish hooks to slay the monster.