The Anti-Monitor is The Monitor’s counterpart from The Anti-matter Universe. He is responsible for the original Crisis on Infinite Earths, was the sole Guardian of The Sinestro Corps and once, against his will, was the source of The Black Lantern Corps Central Power Battery.
The Anti-Monitor debuted in Crisis on Infinite Earths #2 (February 7, 1985).
Crisis on Infinite Earths #2
From the dawn of man to the far-flung future, a mysterious energy is eating its way through the cosmos! To combat the burgeoning disaster, a powerful being called the Monitor recruits a team of heroes from across the Multiverse. Even still, darkforces are gathering their strength in the shadows…