![1girl arm_strap ass back ball_and_chain bare_shoulders boots bracer brown_panties chain closed_mouth contrapposto facing_away from_behind fur_trim holding holding_sword holding_weapon multiple_views nagisa_kurousagi original panties short_hair simple_background sword thigh_boots thighhighs underwear weapon white_background white_hair rating:s score:5 post #3379947](/cdn_image/preview/68/bc/68bca64cad0fbe4104b754a8d7405c9b.jpg)
A heavy and dangerous to use crushing weapon that uses centripetal force for power. It uses a heavy ball-shaped weight like a flail, but the chain is longer and may not be attached to a handle.
A rule of thumb, if the weapon has a handle (especially a long handle) and the chain is under an arm's length, then the weapon is most likely a flail instead of a ball and chain.
For the restraining device, see the ball_and_chain_restraint tag.