The obstacle for Odysseus that is represented in the crossword solution is the Sirens. In Greek mythology, the Sirens were dangerous creatures who lured sailors to their death with their beautiful singing. Odysseus was warned of the danger they posed and had his crew plug their ears with beeswax so they could not hear the Sirens' song. He then had himself tied to the mast of his ship so he could hear the Sirens without being tempted to steer the ship towards them.