What at first appears to be a pile of rust begins to move, revealing itself to be a carpet of russet-colored, insect-like creatures.
Rust Mite Swarm (CR 5)
N Fine aberration (swarm) Init +2; Senses darkvision 60 ft., scent metals 90 ft.; Perception +0
--- AC 20, touch 20, flat-footed 18; (+8 size, +2 Dex) hp 49 (11d8) Fort +3, Ref +5, Will +7 Defensive Abilities swarm traits; Immune weapon damage
--- Speed 40 ft., climb 10 ft. Melee swarm (3d6 plus rust) Space 10 ft.; Reach 0 ft. Special Attacks distraction (DC 15), rust
--- Str 1, Dex 15, Con 10, Int —, Wis 10, Cha 2 Base Atk +8; CMB —; CMD — Skills Climb +10; Racial Modifiers uses Dex on Climb checks
--- Environment any underground Organization solitary, pair, or nest (3–10 swarms) Treasure incidental (no metal treasure)
--- Rust (Ex) Whenever a rust mite swarm deals damage to a creature, any metal armor, shield, or weapon worn or carried by that creature takes an equal amount of damage. A rust mite swarm's swarm attack deals 6d6 points of damage to unattended metal objects. Damage that a rust mite swarm deals to a nonmagical metal object ignores that object's hardness. Scent Metals (Ex) This ability functions much the same as the scent ability, except that the range is 90 feet and the rust mite swarm can only use it to sense metal objects (including creatures wearing or carrying metal objects).
The creatures known as rust monsters begin life as spawn the size of an adult human's thumbnail. These so-called rust mites, which share the four-legged but otherwise insect-like appearance of their forebears, are hatched in the thousands, at which point they gather in ravenous swarms. A menace to mines and other metal-related industries, rust mite swarms prowl the land, consuming metal ore and metal objects by converting the component metal into worthless rust.
Like clockwork, a new generation of rust mites is spawned in rust monster territory every fifty-seven years. The rust mites ravage metal sources across the land, destroying recently opened mines, disrupting trade routes, and toppling economically-dependent civilizations. Only after two years of mayhem do the mites turn upon one another in a ruthless battle for dominance, reducing their legion swarms to mere dozens of individuals who will live to see adulthood.
In many rust monster territories whose inhabitants are short-lived or newly arrived, knowledge of rust mite swarms is lost to the past, or only hinted at in vague prophesies of hardship and loss. Some druidic circles look to such prophesies as the promise of a golden age, one in which the days of iron are undone, the days of stone restored. Such druids actively seek to find and unleash the mysterious catalyst that will usher in this heralded paleolithic revival.