Skitterhaunt Spider* (CR 3)

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

AC: 12 (-1 size, +1 Dex, +2 monstrous vermin shell), touch 10, flat-footed 11
Hit Dice: 4d10+27 (49 hp)
Fort +7, Ref +2, Will -4
Speed: 20 ft., climb 10 ft.
Space: 10 ft./5 ft.
Base Attack +3; Grapple +2
Attack: Bite +4 melee
Full Attack: Bite +4 melee
Damage: Bite 1d8+3 plus 1d6 acid
Special Attacks/Actions: Acid spray, infest vermin
Abilities: Str 15, Dex 13, Con 16, Int -, Wis 1, Cha 1
Special Qualities: ooze traits, sonic immunity
Feats: -
Skills: Climb +10
Advancement: 6-9 HD (Large)
Climate/Terrain: Any underground
Organization: Solitary
Treasure/Possessions: None

Source: Web

Acid Spray (Ex): Once every minute, the skitterhaunt spider can spray a line of acid out from any cracks or openings in its chitinous armor. To do so, the skitterhaunt spider makes a ranged touch attack against a target within 120 feet. If it hits, the target takes 3d6 points of acid damage.

Infest Vermin (Ex): If a skitterhaunt spider damages a living vermin, it can attempt to infest the creature. This is a free action for the skitterhaunt spider, although it can attempt it only once per round. The damaged vermin must make a Fortitude saving throw (DC 15) to avoid becoming infested. An infested vermin takes 1d8 points of temporary Strength damage per day. Once its Strength score reaches 0, the vermin dies and the skitterhaunt spider begins to ooze from its previous shell into the new host's exoskeleton, replacing the creature's internal organs with its own body. This process rebuilds the host creature's lost Strength slowly, at the rate of 1 point per day. During this time, the skitterhaunt is helpless and immobile. When the host creature's Strength returns to its previous value, the skitterhaunt has completely taken over the new vermin's body. It then awakens, fully mobile, and begins to seek out new vermin to infest.

Blindsight (Ex): A skitterhaunt's entire body is a primitive sensory organ that enables it to discern objects and creatures within 40 feet. The skitterhaunt usually does not need to make Spot or Listen checks to notice creatures within range of its blindsight. A silence spell negates this ability and forces the skitterhaunt to rely on its vision (which is as good as a human's).

Sonic Immunity (Ex): Skitterhaunts are immune to sonic damage and effects.

A skitterhaunt spider prowls the tunnels of the Underdark on the constant search for more vermin to infest or for anything else that moves (so that it can eat).