Published in 1983, this was the third hardcover book of monsters for 1st edition AD&D. After the disappointing Fiend Folio, MM2 was a pleasant surprise.

Perhaps most notably, it expanded tremendously on the native beings of the Outer Planes. The Lawful Neutral modrons arrived to balance the Chaotic Neutral slaads of the Fiend Folio, angelic devas populated the Upper Planes, and the Lower Planes welcomed numerous new lords and hordes to plague adventurers.

AD&D rule books