Chilling Fog (CR 9)

Large Ooze
Alignment: Always neutral
Initiative: +1 (Dex); Senses: blindsight

AC: 15 (-1 Size, +1 Dex, +5 deflection), touch 15, flat-footed 14
Hit Dice: 9d10+18 (67 hp); DR: 10/magic
Fort +10, Ref +9, Will +9
Speed: 20 ft.
Space: 10 ft./5 Ft.
Base Attack +6; Grapple +12
Attack: slam +7 melee (1d6+3 plus 9d6 cold)
Special Attacks/Actions: Cone of cold, engulf
Abilities: Str 15, Dex 12, Con 15, Int -, Wis 12, Cha 15
Special Qualities: ooze traits, SR 19
Feats: -
Skills: -
Advancement: -
Climate/Terrain: cold marshes
Organization: solitary
Treasure/Possessions: none

Source: Monster Manual III

Cone of cold (Su): a creature hit by a chilling fog's slam attack or engulfed by it is dealt 9d6 points of cold damage (reflex DC 17 half).

Engulf (Ex): a chilling fog can flow around creatures that fit within its space as a standard action. It cannot make a slam attack during a round in which it engulfs. The chilling fog merely has to move over the opponents, affecting as many as it can cover. Opponents can make attacks of opportunity against a chilling fog, but if they do so they are not entitled to a saving throw. Those who do nor attempt attacks of opportunity must succeed on a DC 17 reflex save or be engulfed; on a success, they are pushed back or aside (opponent's choice) as the spell moves forward. Engulfed creatures take 9d6 points of cold damage (reflex DC 17 half) and are considered to be grappled.

A chilling fog often lurks in misty areas, making it difficult to detect until it attacks.