This flail can be wielded as a magic Martial Melee Weapon that grants a +1 bonus to attack and damage rolls made with it, it also has a 10 foot reach property.

Fractures
After a successful hit with this weapon, the user can use a bonus action to cause a hostile creature who have a skeleton or bone structure to obtain bone fractures, which allows the weapon to deal an additional damage die on hits made with this weapon and this increases with each fracture.

Fractures can be healed from any source of healing including healing kits, spells, and regeneration abilities, but all healing is halved when fractures are present.

Skullcrusher
When striking a creature with 25 or less hit points, that has already received a prior fracture, the creature may be forced to make a DC 15 constitution saving throw or be dropped to 0 hit points.

Proficiency with a Flail allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, creature proficient with martial weapons, Damage, Combat, Sap