Shinobi Slimes Katana is a post skeletron meele weapon that is dropped from the Slime's Last Hope boss.
When enemies are at 10% health or lower, they will receive the Deathblow debuff. This debuff will instakill any nonboss enemy if they get hit by this weapon.
Bosses are immune to the Debuff
Note: if you have the Shinobi Slimes Katana in your inventory, it will give the Deathblow debuff to enemies regardless if you are holding it or not, but will not be instakilled unless you hit them with this weapon.