Thorns Ring

When enemies damage you, they will take damage themselves.
Overview
The Thorns Ring is a craftable ring unlocked at Combat Level 7. Its recipe uses 50 Bone Fragments, 50 

When the farmer is hit while wearing the Thorns Ring, the monster that caused the hit takes damage in return. This reflected damage is based on the attack’s unmitigated damage minus the monster’s defense. If two Thorns Rings are equipped, the reflected damage is doubled. Monsters killed by the ring still drop loot normally, so the ring can finish off enemies without preventing item drops.
The ring also has a use in dyeing. It functions as a yellow dye item at the dye pots in 

- If the damage dealt to the farmer is less than 10, the reflected damage uses the average of the damage dealt to the farmer and the unmitigated damage instead of the usual calculation.
- Kills caused by the Thorns Ring do not count toward Monster Eradication Goals at the Adventurer’s Guild.
Because the ring only triggers when the player is hit, it is most effective in situations where the farmer expects to take repeated contact damage, such as when fighting close-range monsters. It is less reliable as a primary damage source against enemies that attack infrequently or from a distance.
Official description
When enemies damage you, they will take damage themselves.