Rust Monster Swarm CR 3
XP 800
N Tiny aberration
Initiative +6; Senses darkvision 60 ft., scent metals 90 ft.; Perception +12
Defense
AC 17, touch 15, flat-footed 14; (+3 Dex, +2 natural, +2 size)
hp 16 (3d8+3)
Fort +4, Ref +6, Will +2
Defensive Abilities swarm traits
Offense
Speed 15 ft., climb 15 ft.
Melee bite (1 + rust)
Space 10 ft.; Reach 0 ft.
Special Attacks rust, distraction (DC 12)
Statistics
Str 2, Dex 17, Con 13, Int --, Wis 13, Cha 8
Base Atk ++2; CMB --; CMD --
Feats Ability Focus (Rust), Skill Focus (Perception)
Skills Climb +8, Perception +12
Ecology
Environment any underground
Organization solitary, clutch (2-5 swarms), horde (6-12 swarms)
Treasure none
Special Abilities
Rust (su) - A rust monster swarm that occupies a square with any metal in it automatically uses this ability. Any unattended objeccts immediately take half its maximum hp in damage and gains the broken condition--a second hit destroys the item. Against creatures made of metal, this ability does 3d6+5 points of damage. An attended object, any magic object, or a metal creature can attempt a DC 15 Reflex saving throw to negate this effect. The save DC is Constitution based.
Scent Metals (ex) - This ability functions much as the scent ability, except that the range is 90 ft. and the rust monster swarm can only use it to sense metal objects (including creatures wearing or carrying metal objects).
A rust monster swarm is created when hundreds of rust monster eggs hatch. As the young monsters mature, many die or are cannibalized by the stronger in the brood. Their ability to rust metal is just as potent as when they are adults, though they are more able to eat flesh as they grow (as they usually must, since even a large mine wouldn't keep the whole brood fed.)