Another Fiend Folio do-over...
The original version of the Magnesium Spirit is one of a whole dull batch of unillustrated (or under-illustrated) things-that-have-one-exotic-attack-and-then-try-to-possess-your-body monsters in there.
My version here (sorry the bottom bit's blurry 'cause of the scanner fucking up) is a sort of alchemical semiundead. It looks like a sort of awkward, wavery version of a person at first. However, then the upper half of its body explodes in a burst of blinding light (save vs whatever or be blinded) and this flash then throws shadows on the walls or other surfaces nearby.
If the flash of light should happen to hit any living thing, its shadow will turn into a shadow with abilities and hit dice commensurate to the size of the cast shadow itself (in most cases this will be limited by the size of the room.) Outside in mist, the creature's flash will create Brocken spectres--which behave as wraiths.
As these shadows attack, the Spirit itself will attempt to gnaw at the target(s) in order to consume the magnesium in their bones. Its physical attack is relatively feeble--however, after 4 rounds it will regenerate its upper body and be able to generate another flash.