The Amulet of First Magic was a shiny, purple amulet that, according to Morgaine Le Fay, was the source of all Earthly magic. It possessed incredibly powerful magic, so powerful that it appeared to be able to do literally anything its master willed it to do. However, its power was not infinite.
Morgaine and Mordred searched for it in the ancient ruins of a building. When Mordred got a hold of it, he betrayed his mother, and used its power to banish everyone that was older than him to a shadow realm, and transformed an amusement park into his own kingdom. With the help of Morgaine's magic, Superman, Batman, Wonder Woman, and Green Lantern were able to return to Earth in the form of children. They confronted Mordred, but he was too powerful. Soon after, the junior Justice League members team up with an infant Etrigan. After a long battle, the amulet was destroyed by Etrigan when he tries to eat it. Batman then goads Mordred into using up all of the power he absorbed from the amulet to break the spell keeping him eternally young, reversing all of the spells he cast in the process.