Between the Monster Manual of AD&D and the Monstrous Compendium of 2^nd edition AD&D, there are ten years of the game being played. There were some notable changes in the new monster lists, with quite a few omissions due to the Angry Mothers from Heck problem that was plaguing AD&D at the time; in particular, demons and devils don’t appear in the original two Monstrous Compendiums, although they would be restored using a new name in later supplements. More significant from the perspective of how the game played, Giants and Dragons were made significantly tougher, primarily in the number of hit dice they had.

I’m looking at the compiled Monstrous Manual for these values, partly because it’s a snapshot of the design a few years later, but mainly because it isn’t falling to bits like the binder-format of the Monstrous Compendium is!

It’s notable to see how little Armour Classes have changed.

Dragons gain much better armour classes. This time around, they have a separate Armour Class for every age category – all twelve of them – and so the Black Dragon goes from an AC of 4 as a wyrmling to AC -7 as a Great Wyrm (compared to AC 3 in first edition). The Red Dragon even breaks the “-10” barrier, reaching a total of -11 at its highest age level!

A few very dangerous and high level monsters get good ACs: the Composite Elemental Skriaxit (16+16 HD) has AC -5, the Great Feyr (16 HD) has AC -2.

Giants have also improved their Armour Classes. Fire Giants drop from AC 3 in first to AC -1 in second. Hill Giants only drop one point to AC 3. Frost Giants drop four points to AC 0. The Storm Giant now has an AC of -6 (from 1).

Demons and Devils also improve – the Balor is now AC -8 from -2, and the Pit Fiend -5 from -3. The extent of the changes isn’t actually consistent. Plenty of monsters, for instance, Orcs, Ogres, and Trolls, have not changed at all. It is only a very few where the designers thought the previous versions were too weak that have improved. The vast bulk of the monsters are unchanged, and retain ACs in the 2 to 9 region.

Meanwhile, the game has changed with respect to the players. Weapon Specialisation has become enshrined in the rules, and the combat tables have slightly changed. It’s not a big change, but higher level characters are slightly better at hitting creatures.

What you end up with is a system where Armour Classes are, for the most part, still bounded. Hit Dice still are the most significant indicator of the difficulty of a monster. High level characters will normally be able to hit monsters, so damage vs hit points is quite important, but every so often a monster will come along where only the fighters and spell-casters are likely to work well against it; the thief is somewhat out of luck.