The terithrans are ugly, hairless humanoids with enormous ears found on the Ethereal Plane. They sleep peacefully in tombs of solid ether until disturbed by the presence of arcane magic used on a plane coexistent with the Ethereal. Strangely, a terithran ignores magic used on the Ethereal Plane itself, but tries to kill an offending Prime-based mage. The higher the level of the arcane spell cast (and the higher the frequency of arcane magic use) in an area coexistent with a terithran’s tomb, the greater the chance of disturbing the terithran. If disturbed, the creature angrily roams the Ethereal for weeks after dealing with the offender.
Terithrans are enemies of the Garmorm, though no one knows why.
Source
Planescape Monstrous Compendium Appendix III
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