Actually, they are not "new" at all. The Shadow Elves were the stars of a Basic D&D module waaaay back when race was the same as character class, and all maps used wiggly lines to represent curtains. My lads have recently been playing Shadows over Mystara and have been suffering the predations of the Shadow Elves, and I remember writing a bit many years ago about how they existed in some super deep realms of my world.
|Clothing optional apparently....|
So, what follows is the sub-race description for turning your elf PC into a Shadow Elf (or Glwyrr'Syth in my world)
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GLWYRR'SYTH (SHADOW ELF) SUB-RACE
Ability Score Increase: +1 Intelligence
Superior Darkvision: You can see perfectly in total darkness and shadowy conditions, up to 120'.
Born of the Deep Fires: The Glwyrr'Syth tend to live near areas of volcanic activity, and have an innate Resistance to Fire
Born to the Shadows: Glwyrr'Syth are naturally stealthy. They are automatically proficient with the Stealth skill, and may add twice their proficiency bonus to Stealth checks.
Cavern Born: Glwyrr'Syth cannot become lost in natural cavern complexes or similar areas, and always know which way is North.