10 kills to charge
When you use it, your next attack will halve the targeted enemy's current HP, but also halve the damage of the weapon used **until the enemy is killed. **
So, for example, if you are fighting a monster with 10/24 HP, it will halve the remaining, resulting in 5/24 HP (if we assume your weapon didn't deal any additional damage, in that case, halving would damage the enemy first, then your weapon damage)
This is almost exclusively useful against bosses and at the very beginning of their fight, since you can instantly chip away half of their HP with your less useful weapon, and then kill it with your main, unaffected weapon.