The Onyx Boot is an accessory that adds 3 points to your Agility Skill, thus allowing you to "travel faster, jump higher, and maximize your manual dexterity."
How to Obtain
This item can be purchased from the Crown Store, where it's found in the "Tools And Accessories" section. You can also trade for it in a market world. Rumor has it that it's also possible to loot this item from a mechanical chest, but to date there have been no confirmed cases of this actually happening.
How to Use
To equip this item, drag it to an active slot in your accessory bar.
Do not confuse this item with the Onyx Stocking, which is a purely decorative, albeit rare, holiday item that will NOT boost your agility skill.