I wonder though, if/when you get a rust monster of the appropriate int that it can take class levels, does the other stuff really apply any more? I mean, if it's smart enough to do reasoning, does teaching it tricks by reward and punishment conditioning still viable? Or will it decide to screw you, and go out and earn food on its own merits as the most annoying adventurer ever?
You are correct, once Intelligence is boosted, "training" details are no longer necessary.
No sane DM would permit such a thing. No sane player would try it and not expect the DM to take control of the situation quickly.
For those outside the realm of sanity, I'd like to imagine a bargain between player and DM could be arranged, such as what Jackinthegreen has proposed.
Please understand, my guide is designed to offer as much as possible of
what could be according to RAW. It is in no way suggesting
what should be attempted. With the exception of the last section "
Thinking Outside the RAW", I have tried to keep everything strictly to the letter of the D&D law so as to make it a viable option for any players and DMs, without any houserule fluff, or confusions.
One could view it like a Wizard's relationship with his familiar: Both gain something by helping the other. In the case of the Rust Monster, it gets some love and attention in
addition to usually getting a nice fill of metal items. It'll be intelligent enough to realize that it probably wouldn't get the same opportunities on its own. The character who raised the rust monster gets the benefits of a friendly, trained, metal-eating mount and/or companion.
Yeah, this is a possible way to arrange a compromise between an eager player and a patient DM.
In my imagination, an intelligent Rust Monster would reason that ultimately he/she knows the way to best feed his/her hunger, and would proceed first to flee safely, gain class levels in something that would allow the acquisition or creation of more Headbands of Intellect, than enhance the next generation and multiply the process. Shortly after, a
Plane Shift to The Infernal Battlefield of
Acheron occurs, where the small army grows into an Empire. They expand over to the Clockwork of
Mechanus and eat the gears that run the rest of the Multiverse.
But hey, that's just me.
Oh, by the way... One can give the Celestial template to a Rust Monster, since Celestial can be added to any corporeal aberration, etc of good or neutral alignment.
The objective of my guide is to include clear (or mostly clear) Rules As Written, ways to do anything with a Rust Monster, that would be possibly beneficial to a PC.
While there are many, many templates that
could be applied to the Rust Monster, as per any DM's creation of any creature for whatever reason, the templates I have listed are ones a single PC could influence within a campaign.
To the best of my knowledge, no planar templates can be applied by any RAW process to an individual creature (Although I feel like I read in one of the shadow/dark templates something about extended exposure to the Plane of Shadow can turn a creature to acquiring the template, but I can't find that wording anymore. If anyone knows if/where it ways that, I'll add it), or be bred in during a human's lifespan. Now, for the sake of RP, a story could be made up that a series of generations of Rust Monsters bred on the Peaceable Kingdoms of
Arcadia might eventually yield descendants that would possess the celestial template. However, that's some fluff discussed between a player and a DM, and outside the scope of what my guide addresses.