Emblem of Monster Slayer
Whoever fights monsters should see to it that in the process he does not become a monster.
It can be created in a Crafting Table, via shaped recipe:
When worn in the Charm slot, it will give the wearer +25% Damage against undead creatures, +10% Damage against other agressive creatures, +1 Looting Level, and will double amount of experience dropped from any monsters.
Only one of two damage bonuses will apply to a victim of emblem bearer. If victim is undead, it will receive additional damage based on bonus against undead. If it's not, but it's still an agressive creature - then second one.
Definition of "agressive creature" excludes not only peaceful, but also neutral mobs such as Enderman and Zombie Pigman, and few more, less obvious types - for instance, Guardians and Blazes.
At the same time, experience bonus will apply to anything counted as monster, and bonus looting level will apply to anything without exceptions.
All of the charm bonuses work regardless of player attacking their targets directly or indirectly - as long as damage dealt is considered a damage from this player.