Nightmare Rat Working Draft
Nightmare Rat
Tiny Magical Beast
Hit Dice: 1d10 (5 hp)
Initiative: +2
Speed: 40 ft. (8 squares), climb 20 ft.
Armor Class: 14 (+2 size, +2 Dex), touch 14, flat-footed 12
Base Attack/Grapple: +1/-11
Attack: Bite +5 melee (1d3-4 plus phantasmal killer)
Full Attack: Bite +5 melee (1d3-4 plus phantasmal killer)
Space/Reach: 2½ ft./0 ft.
Special Attacks: phantasmal killer
Special Qualities: Low-light vision, scent
Saves: Fort +2, Ref +4, Will +1
Abilities: Str 2, Dex 15, Con 10, Int 2, Wis 12, Cha 15
Skills: Balance +10, Climb +12, Hide +16, Move Silently +10, Swim +10
Feats: Stealthy, Weapon Finesse (B)
Environment: Any
Organization: Solitary, family (2-10), or pack (11-20)
Challenge Rating: 1
Advancement: —
Level Adjustment: —
A giant rat of peculiarly vile appearance. It has an air of malevolence that is almost fiendish.
Nightmare rats are supernatural entities who occasionally appear in  areas where a dream dimension incurs into the material world. Some  scholars believe they are an illusory manifestation of the fear of  rodents, rather than "real" physical creatures. If these nasty little  monsters are material creatures, they are most likely the mutated  descendants of ordinary rats that somehow crossed over into a dimension  of dreams.
A nightmare rat can grow to be 2 feet long and weighs up to 8 pounds.
COMBAT
Nightmare rats attack from ambush, trying to bring down prey with their phantasmal killer ability. They flee if confronted, but fight viciously when cornered or defending their nest.
Phantasmal Killer (Su): Any creature bitten  by a nightmare rat immediately sees an image of an incredibly fearsome  beast. The victim must make a DC 12 Will save. If they fail, they take  3d6 damage and must succeed at a DC 12 Fortitude save or die of fear.  This vision can never be reflected on the nightmare rat, but it acts  otherwise like a phantasmal killer spell. This phantasm is a mind-affecting fear effect.
Skills: Nightmare rats have a +4 racial bonus on Hide and Move Silently checks, and a +8 racial bonus on Balance, Climb, and Swim checks. A nightmare rat can always choose to take 10 on Climb checks, even if rushed or threatened. A nightmare rat uses its Dexterity modifier instead of its Strength modifier for Climb checks.
Originally appeared in Tales From the Infinite Staircase (1998).