Shadow Tunic is a type of Clothing that is worn in the Coat slot. It is received as part of a set together with 
Dark Pants in exchange for giving an Orange 
Prayer Coin to the Torch Dragon Merchant. They provide the highest possible Temperature Tolerances of any type of Clothing, and breakthroughs 1 Skill Level of Spell[1].
The item is gender-locked to only male characters[2], which makes it less useful as Sub-Spirit Gear unless the sect is intentionally made single-gender. Specter Tunic is the identical female version of the item.