You are granted a mystical cowl made of otherworldly material from your warlock patron. While wearing this Cowl and no other armor, you gain an AC of 10 + your Charisma modifier + your Dexterity modifier.
The Cowl is physically bonded to you, and you can equip it as a bonus action and dismiss it without any action required. If you lose the Cowl, you can perform a 1-hour ceremony to receive a replacement from your patron. The Cowl disappears if you are killed.
You can transform one magic cloak, cape or robe item into your Cowl by performing a special ritual while you are holding the item. You perform the ritual over the course of an hour, which can be done during a Short Rest. You can then dismiss the magic item, shunting it into an extradimensional space, and it appears whenever you create your Cowl thereafter. The item ceases being your Cowl if you die, if you perform the 1-hour ritual on another magic item, or if you use a 1-hour ritual to break your bond to it.