After getting 10,000 or more race kills you can speak to an NPC in the Demon realm to gain a bane item. Bane items can be applied to weapons and can be used in addition to poison/weakness/imprints.
For reaching 10,000 kills the bane item you are given will allow you to deal 5% more damage to that race when applied to your weapon.
This allows you to do much greater damage to that specific race and will also work for things like PvP when fighting a player of that specific race.
To gain a bane item go speak to the NPC in the Town of Path inside the Demon Realm.
Applying Bane to a Weapon
To apply bane to a weapon simply use the bane item from your inventory then choose the weapon to apply it to.