Phase Spider (CR 5)

Large Magical Beast
Alignment: Always neutral
Initiative: +7 (+3 Dex, +4 Improved Initiative); Senses: darkvision 60 ft., low-light vision, and Spot +4

AC: 15 (-1 size, +3 Dex, +3 natural), touch 12, flat-footed 12
Hit Dice: 5d10+15 (42 hp)
Fort +7, Ref +7, Will +2
Speed: 40 ft., climb 20 ft.
Space: 10 ft./5 ft.
Base Attack +5; Grapple +12
Attack: Bite +7 melee
Full Attack: Bite +7 melee
Damage: Bite 1d6+4 and poison
Special Attacks/Actions: Poison
Abilities: Str 17, Dex 17, Con 16, Int 7, Wis 13, Cha 10
Special Qualities: Ethereal jaunt
Feats: Ability Focus (poison); Improved Initiative
Skills: Climb +11, Move Silently +11, and Spot +4
Advancement: 6-8 HD (Large); 9-15 HD (Huge)
Climate/Terrain: Warm hills
Organization: Solitary or cluster (2-5)
Treasure/Possessions: None

Source: Monster Manual

Ethereal Jaunt (Su): A phase spider can shift from the Ethereal to the Material Plane as a free action, and shift back again as a move-equivalent action (or during a move-equivalent action). The ability is otherwise identical with ethereal jaunt cast by a 15th-level sorcerer.

Poison (Ex): Bite - Fortitude save (DC 15); initial and secondary damage 2d6 temporary Constitution.

Phase spiders dwell and hunt on the Material Plane.

Once a spider locates prey, however, it shifts to the Ethereal Plane to attack, attempting to catch its victim flat-footed. The spider shifts in, bites its victim, and retreats quickly back to the Ethereal Plane.