The Eternity Glass is a legendary hourglass with the power to bend space and time. It also served as the power source of the Time Trapper.
At some point, he was imprisoned and bound to the Eternity Glass.
In the 31st Century, the Eternity Glass was sealed behind protective glass at a museum exhibit. Lex Luthor stole the object and while trying to figure it out, released the Time Trapper by turning it upside down. Still bound to the hourglass, Trapper gave Luthor and the group of his fellow villains known as the Legion of Doom a choice. Luthor returned to the past and planned to use the Eternity Glass to erase Superman and the Justice League for good.
Karate Kid and Dawnstar sought out Luthor in the present and awakened him. The Luthor from the future was now a paradox and erased by Trapper. Time Trapper took control of the Eternity Glass and generated a giant portal to remake the world in darkness. Dawnstar used her light energy and rammed into the Time Trapper. He dropped the hourglass in the explosion. Robin retrieved it as Batman, Wonder Woman, and Superman held off Trapper. Karate Kid instructed him to turn the hourglass. Time Trapper was imprisoned once again and the giant portal was dispersed.
Karate Kid and Dawnstar used the Eternity Glass to return to the 31st Century but discovered the timeline was not fully restored (as it shows a "commemorated" statue of Lex Luthor at the museum). They used the hourglass to return to the past.