The Earth-15 dimension was the so called the "Perfect Universe". Destroyed during a rampage of Superboy Prime, millions of lives were lost. The dimension still existed but the delicate structure of spacetime was irreparable damaged. Only one fragment of this universe survived: the sole relic of Earth-15 itself was the Cosmic Grail, which would survive the events of Flashpoint. It said is hidden in one of the many worlds of the multiverse.