Unfamiliar with Krona? He is a summary of history within the DC Universe:
Born on Maltus, Krona is an Oan scientist who became obsessed with observing the origins of the universe itself, despite an ancient legend that said discovering that secret would cause a great calamity. Krona constructs a machine that pierces the temporal barrier and views the beginnings of time itself. Krona catches a glimpse of the hand of creation depositing the speck that would become the entire cosmos. His machine explodes at that instant, and the universe is shattered causing the creation of the Multiverse as the universe replicates into an infinite number of parallel universes. At this same moment, the Anti-Matter Universe is created, unleashing evil into the cosmos; thus, the Monitor and the Anti-Monitor are born. As punishment for this act, Krona is transformed into pure energy by his fellow Oans and is sent to forever wander the cosmos. Krona's actions are what caused the Oans to become the Guardians of the Universe.