Aion is a persistent world online RPG set in an ethereal environment where players can join either side of the antagonistic battle between angelic and demonic creatures. In addition to a classic struggle between good ("The Chosen") and evil ("The Fallen"), gamers on both sides must also deal with a server-controlled NPC faction known as The Balaur, which the developers included as a balance in the event one side becomes too powerful. Aion runs on Crytek Studios' proprietary game engine, which allows the game to employ dynamic temperature and weather effects (including special weather-based attacks) and realistic environmental characteristics. As in most MMOs, characters can be drawn from a variety of different classes, and players grow and level up through combat, economic advances, and social interaction.