Ny'ghan Grii

The Ny'ghan Grii are a hostile alien species from another dimension, which invaded Earth thousands of years ago, but were defeated by "the gods friendly to mankind", such as Vorvadoss. The occult book De Vermis Mysteriis by Ludvig Prinn refers to them as the Dwellers in the Hidden World.

Biology
The Ny'ghan Grii are white translucent creatures. Their bodies are nearly spherical, flattened at the top and the bottom, and covered in long thin tendrils. Their faces consist of a large faceted eye and a small orifice below it, which might be a mouth. They make bird-like vocalizations and their presence has some effect on the environment that causes temperatures to drop noticeably whenever they're nearby.

Most notably, the Ny'ghan Grii have the supernatural ability to "devolve" other creatures, causing their bodies to regress to a form resembling their evolutionary ancestors. For instance, a Human victim might become simian, then reptilian, then dissolve into amorphous ooze.

Appearances

 * "The Invaders", by Henry Kuttner (1939)