Grues
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(From Complete Arcane - 2004 - [credits]):
Grues are horrible creatures spawned in dark places within the Elemental Planes, the result of the taint of evil magic. Eager to leave their home planes at any opportunity, these creatures are prized by many wizards, for at the heart of each grue lies a pearllike magic object the size of a fist, each carefully etched with the arcane workings of an elemental spell suitable for copying into a spellbook. This spell object's origins lie in the grue’s own magical beginning, and is the only thing that remains of the creature if it is slain. See the individual elemental grue descriptions for details.
All four types of grues can be summoned by means of a summon monster IV spell (the spell acquires the evil descriptor when so used). Note that summoned creatures return to their home planes if killed on the Material Plane, so a summoned elemental grue slain on the Material Plane leaves no spell object behind.
Combat: Each type of grue has its own unique combat abilities and tactics, but all share some qualities, An elemental grue's natural attacks, as well as any weapons it wields, are treated as evilaligned for the purpose of overcoming damage reduction.
Spell Disruption (Su): The very presence ofan elemental grue interferes with spells that affect the grue's associated element. Any spellcaster within 40 feet of a chaggrin who casts a spell with the same descriptor as the grue’s element (earth, fire, air, or water) must succeed on a DC 15 caster level check or have the spell fail. Within the same area of any such spell currently in effect, a grue has a chance to dispel the effect as a free action, as if casting a targeted dispel magic (caster level 10th).
(from Planescape: Monstrous Compendium Appendix I - 1994 - [credits])
Grues are underling creatures of the Elemental Planes, more important in the ecology than vermin but less intelligent than true elementals. They occupy a niche somewhat equivalent to animals, in other respects more like servants, that has no exact equivalent on the Prime Material Plane. Grues prety on vermin, serve as pets and guards for elementals on their native planes, and can be conjured to other planes by a magical summons.
The four known varieties of grue have type names (chaggrin, harginn, ildriss, varrdig), but planars usually refer to them as "earth grues," "air grues," and so on.