eidolon

eidolon \ahy-DOH-luhn\, noun:

1. A phantom; apparition.
2. An ideal.

"An eidolon, a manifestation, if you will, sent up to us from the uttermost deeps to bring about the end of the world."
-- Neil Gaiman, Looking for the Girl

It was the ghoulish shade of decay, antiquity, and desolation; the putrid, dripping eidolon of unwholesome revelation, the awful baring of that which the merciful earth should always hide.
-- H.P. Lovecraft, The Outsider

Eidolon comes from the more common word idol, which originally referred to pagan religious idols. It was first used in the 1820s.