The rust monster is one of those D&D monsters that is interesting because it requires different tactics than usual.
Well, it has the potential for that to make it interesting. When the PCs encountered one in my Skylands campaign, standard tactics worked just fine, and the PCs never saw its special ability in action. The character with the highest AC—who might have been reluctant to attack if he’d known what a rust monster was—took it on essentially by himself. The rust monster never scored a hit.
The rust monster presents two dangers: One to metal armor and one to metal weapons. Let’s look at weapons first.
I was using the Rules Cyclopedia description, which says...
A successful attack roll indicates that the rust monster’s body is hit, which does not harm the weapon.
I see now, however, that Moldvay wrote...
If a character hits a rust monster, or if a rust monster hits a character with its antenna, it will cause any metal armor or weapons touching it to immediately rust
If I had used the Moldvay description, the players would have discovered the danger to weapons.
We could do a middle-ground approach. Only the antenna cause rust, but give each attack against the rust monster some chance of hitting the antenna instead of the body. And allow anyone who is specifically avoiding the antenna to do so with at a penalty.
But that’s more complication than I want. So I’ll run rust monsters the Moldvay way in the future.
Aside: So often when I find a difference between another edition (or a retro-clone) and Moldvay/Cook/Marsh D&D, I prefer the latter. It’s the organization, though, that has me using the RC and the Creature Catalog at the table.
To me, the rust monster’s danger to armor, however, is the more interesting aspect. Once the danger is realized, it is easy enough for the PCs to change weapons. It is harder to change armor. But that danger is never realized if the rust monster can never land a hit.
I’m not a big fan of “touch AC” as a generalized rule. I want characters to have a single armor class as much as possible.
Using “touch AC” as a special rule for a specific monster, however, I think fits in the classic D&D style. So I’m liking the idea that a rust monster’s attacks ignore armor.
(Thanks to everyone one G+ who participated in the discussion about this back in December.)