Extradimensional Lurker - By Bill Silvey

Frequency: Very Rare
No. Appearing: 1
Armor Class: -3
Move 6"
Hit Dice: 7
% In Lair: 0%
Treasure Type: Nil
No. Of Attacks: 1
Damage/Attack: See Below
Special Attacks: See Below
Special Defense: Can only be hit/damaged by +1 or better weapons
Magic Resistance: Standard
Intelligence: Genius
Alignment: Chaotic Evil
Size: L (7')
Psionic Ability: 180
Attack/Defense Modes: All/all
Level/XP Value: VIII/775 + 8 per hit point

Any time an extradimensional space is created - either through a Maze, Rope Trick, or other magical means there is a chance it's creation might be noticed by the Extradimensional Lurker. This beast is similar in appearance to an Intellect Devourer (so much so it cannot be denied that they share a common lineage). 

If the Extradimensional Lurker notices the creation of an extradimensional space by any means (1% chance), then it will use a special form of probability travel to enter that space and attack the occupants therein, consuming the dying scream of their psyche for nourishment as it uses psionic blast to assail the occupants.

It is rumored that the weird and malevolent Githyanki have occasionally formed alliances with these dreadful creatures.

The Extradimensional Lurker's outline is very hard to discern; it has a Displacer Beast like quality that makes it appear to not be where it is. This gives it it's low Armor class, and also makes it very, very resilient to weapon damage. So much so that it cannot be injured by normal weapons.

Extradimensional Lurkers are fond of spaces created via maze spell; they seem to enjoy stalking their prey through the twisting corridors and planes of force that comprise that spell. 

If an extradimensional space is created on the Astral Plane, the probability that it will become haunted by one of these creatures is increased to 5%, and must be checked every turn that the zone exists. If the extradimensional space is Dispelled or if the spell itself expires, the Lurker will be cast back to whatever strange zone it originated from.