Borean Armor Kit

Permanently increase the Stamina of an item worn on the head, chest, shoulders, legs, hands or feet by 12.

Attaching causes the item to become soulbound.

Armor kits fill the same virtual "slot" as enchantments and will overwrite any existing enchantments, or be overwritten by enchantments applied later.

Source
Borean Armor Kit can be crafted by Leatherworkers with a skill level of 350.