Ryoko's Guide to the Yokai Realms, pg 219
Prerequisite: 8th Level
You have practised extensively with magical prostheses, training your mind and body to wield their magic as naturally as you breathe. You gain the following benefits:
• You are considered proficient with any attacks made using a prosthesis.
• You can attune to one magical prosthesis without counting against the number of magic items to which you can attune.