Probably the most controversial monster since the release of Monster Manual 2, the rust monster gets its very own Ecology article. It goes through the history, touching on origins that ranged from a wizard to natural evolution, then moves on to physiology (they might be mammals!) and habitat. In addition to extensive information on rust monsters, there is also a trio of new alchemical items and variant rust monsters. My favorite of the three is the rust bomb, which inflicts an AC penalty to targets wearing metal armor. Unfortunately, the penalty is set across the board. I would have liked to see it increase a bit at higher levels.
- The rust monster lodestone is what you get when the critter eats too much highly magnetized metal. It (thankfully) loses the ability to rust items, but can still corrode rusting items. Its main feature is its ability to slide and slow targets via magnetic pulses. Hell, one of its attacks lets it launch an ally at an enemy, granting the ally a free basic attack with a bonus.
- Terrors are rust monsters that are somehow imbued with the fear of losing your stuff. Not much changes, but the rust monster can use a fear effect that immobilizes targets. Also, if you nail it with a power that has the Fear keyword you both take ongoing psychic damage. Interesting...
- Finally, the nightmare. This is the rust monster big gun, a level 24 solo controller. Its got a laundry list of a stat-block that compares to a dragon: melee corroding attack, ranged corroding attack, can shift and make a shitload of basic attacks (this is an at-will, by the by), a recharging ranged attack that stuns/knocks people unconscious, close blast that immobilizes everyone, can stop teleporting (and gain temp hp to boot), and finally can cause any magical item used to attack it to start to decay (even non-metallic implements). So...yeah. Be very afraid.
A pretty comprehensive article. There are some minor editing quibbles, but nothing major. It goes through the motions and brings a lot of crunch to the table, and it satisfies both the story and mechanic extremes of the spectrum.