Proposed Canon
First Appearance: TBFGK 21
Shady Elves are an Elf brand.
Shady Elves seem to be an analog of "Dark Elves" from other sources. Their dark gothic appearance leaves their motives in question - they look more like Wanda than anything. Likely the "bad boys" of the elves, they are probably the fiercest fighters other than the Eager Elves, and seem to be effective archers, likely second only to the Woodsy Elves. They also probably have access to certain dark mancies (Naughtymancies?) that the other elves don't (like Croakamancy). They may also have access to veils, obfuscation, and other concealment-related forms of Foolamancy.
 Real World References
Shady Elves seem to dress like goths.