Note for GMs: The following item has been made intentionally stronger, this is because in its original printing the author put a high value on the challenges that were required to attain it. Caution is advised when granting it to player characters.
Prankster’s Club
Weapon (greatclub), very rare (requires attunement)
Though it is called a prankster’s club, only a twisted mind would think this weapon is funny. It is magically lightened, making it easier to swing, and enchanted to inflict tremendous pain on impact.
You gain a +1 bonus to attack and damage rolls made with this magic weapon. When you hit a creature with this weapon you can use your bonus action to make one additional weapon attack against the same target. If this attack hits, the target must succeed on a DC 15 Constitution saving throw or suffer an internal injury. While injured the target can take only an action or a bonus action on its turn, but not both, and cannot take reactions. Once you’ve hit with a bonus action attack from this weapon, you cannot use that property again until you finish a short or long rest.
Ref: T&H p166