Elysion (エリュシオン) was the location of the Golden Kingdom of the Earth. In the Silver Millennium, this was the kingdom from which Prince Endymion came. The guardian of Elysion was Helios, who was assisted by two Maenads. Elysion was destroyed with the attack of the Dead Moon Circus.
In the anime, Elysion was first revealed to Chibiusa in her dream in episode 158. In that episode, Pegasus took Chibiusa (temporarily grown up because of a spell by PallaPalla) to fly over the devastated kingdom in order to reawaken her true dream. Mamoru was taken to Elysion in episode 165, when the Sailor Senshi were fighting with Queen Nehellenia. When he arrived, he was transformed into Prince Endymion and entered the castle. Helios explained who he was and apologized for not being able to protect the Golden Crystal.
Elysion's name is taken from the Elysion (also spelled "Elysium") of Greek mythology, which was the part of the Underworld in which the souls of the heroic and virtuous would rest in a peaceful paradise.