Undead Armament (5e Feat)
From D&D Wiki
Thanks to practice, you are now capable of attaching and utilizing extra arms!
As a bonus action, you can attach or detach an additional arm to your body, and can have up to two extras on you at a time. These arms always count as offhand for the purposes of dual wielding. While you have at least one extra arm, you have advantage on any checks made to climb. You cannot attach new arms or wear armor while you have extra arms if the armor you are wearing or trying to wear does not accommodate for your extra limbs.