Spectre of the Brocken
The Brocken is the highest summit of the Hartz mountains in Hanover. This summit is at times enveloped in a thick mist, which reflects in a greatly magnified degree any form opposite at sunset. In one of De Quincey's opium-dreams there is a powerful description of the Brocken spectre.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894