If you were mountain climbing, at a time of day when the sun was low and behind you, and if you climbed high enough to look down into a mist below you, you might witness the shadowy figure of the Brocken Spectre.
It’s your own shadow that you see, cast on the surface of the mists below, surrounded by a halo-like ring of light. The sun must be behind you. You’re seeing your shadow projected in front of you, through the mist.
The Brocken Spectre is a type of glory. Glories are often seen by air travelers, who see the shadow of their airplane cast on clouds below. The airplane’s shadow will be moving along on the cloud tops, as the plane speeds through the air. It’ll be surrounded by light.
That’s also true of a with a mountain glory, or Brocken Spectre. You’ll be standing with the sun at your back, gazing at your own haloed shadow cast on fog.
The Brocken Spectre is also called a Brocken bow or mountain spectre. It takes its name from the Brocken, a peak in the Harz Mountains in Germany. This region is known for frequent fogs. Johann Silberschlag, a German Lutheran theologian and natural scientist, is said to have described the Brocken Spectre in 1780. It has since figured in literature about the region and elsewhere.
At times, your shadow in a Brocken Spectre will appear enormous, but this is an optical illusion. At his great website Atmospheric Optics, Les Cowley explains why the shadow of the Brocken Spectre can appear so huge:
The spectre sometimes appears to be huge. This is probably caused by the presence of the glory and the mist obscuring more familiar reference points with which to judge its size.
Bottom line: The Brocken Spectre is your own shadow cast on mists below you, when you are mountain climbing. The shadow may appear enormous and has a luminous ring around it.