The Compendium has it listed as an aberration, but I felt that it could also be an elemental creature, so I listed it as such, since that is how I started writing it before I found that listing.
Large elemental, lawful neutral (shapechanger)
Armor Class 16
Hit Points 76 (8d10+32)
Speed 30 ft.
STR DEX CON INT WIS CHA
16 (+3) 14 (+2) 18 (+4) 11 (+0) 13 (+1) 10 (+0)
Damage Resistances (whilrwind form only) bludgeoning, piercing, and slashing from nonmagical weapons
Condition Immunities exhaustion, paralyzed, petrified, poisoned, unconscious
Spell Immunities The glasspane horror is immune to all illusion (pattern) spells, spells with the words color or prismatic in their names, and spells with the light subtype.
Senses tremorsense 60 ft., passive Perception 11
Languages Terran, telepathy 60 ft.
Challenge 3 (700 XP)
Shapechanger. The glasspane horror can use its action to polymorph into a humanoid creature seemingly hewn from scintillating gemstones, a sheet of clear glass, or a whirling funnel cloud of stinging, crushed gems. Its statistics are the same in each form.
False Appearance. While the glasspane horror remains in it’s sheet of glass form, it is indistinguishable from an normal window.
Blinding Light (humanoid form only) Once per round as a standard action, the glasspane horror can emit a burst of light that blinds all creatures within a 30-foot spread who fail a DC 15 Dexterity save. This blindness lasts for 1d4 minutes.
Air Form. (funnel cloud form only) The glasspane horror can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.
Multiattack. (humanoid form only) The glasspane horror makes two slam attacks.
Slam. (humanoid form only) Melee Weapon Attack: +5 to hit, reach 5 ft., one target.
Hit: 7 (1d8 + 3) bludgeoning damage.
Trample. (funnel cloud form only) Melee Weapon Attack: +5 to hit, reach 5 ft., one target.
Hit: 13 (2d6 + 6) bludgeoning damage
A pane of glass nearby suddenly cracks and appears to crumble. As the pieces of glass tumble they stop in mid-air, rising slowly and beginning to form into a glittering whirlwind.
Glasspane horrors make excellent guardians, and many powerful wizards and nobles seek them out as guards. The horrors consume gems to survive. Thus, a wealthy merchant can offer a glasspane horror a steady diet of high-quality diamonds, emeralds, and so forth in exchange for its service. Once a horror enters a contract, it does its best to live up to its bargain. A glasspane horror might wait patiently within its hidin
g place for days. While in its glass form, it is almost utterly inert. It can survive for years without feeding.
While glasspane horrors are faithful servants, they are by no means bound to give their lives for their masters. A severely injured (10 hit points or less) horror flees to warn its master of any intruders. If a horror's employer is slain, the creature is freed from its bond. In many cases, a glasspane horror's agreement with a master includes a stipulation that it is to hunt down and attack those responsible for the master's death. The horror does not fight to the death to fulfill this agreement. It usually flees if severely injured, but it does do its best to launch at least one ambush or attack.
If encountered while between masters, a glasspane horror attempts to parlay with intelligent creatures. It offers its services as a guard in return for gems, although it cares little for work that does not allow it to rest in place and guard a static position, treasure, doorway, and so forth.
Glasspane horrors prefer to attack from ambush. When hired by a master, it usually insists that it be given an appropriate place to assume its inert form and ready itself to attack intruders. The horror might take a position with a glass case used to store a treasure, a skylight situated above an important item or doorway, and so forth.