Lion turtles are the creators of bending. There was one for each element. Wan, the first Avatar, asked each for the power of their element, in order to fight Vaatu, the dark spirit, with the help of Raava, the light spirit, in order to restore balance. In Avatar Aang's time, it seems that there is only one lion turtle left. It granted him insight into spirit bending, allowing him to become fully realized, defeat Fire Lord Ozai, and end the Hundred Year War.