The Poison Staff is typical staff, crafted with a poison essence and staff (Shapeless). When attacking or right clicking with the staff it will apply poison II for 15 seconds, dealing 25 hearts of damage for the duration, the activation of the ability cost only one durability always. Of course, the poison debuff ignores defense, making it useful for armored targets, such as those with shields or the Walker Boss.
The right click ability is ranged and if used on a hostile or neutral outside of their activation range, they will take damage to the debuff if they are allowed (Skeletons do not take poison damage) and will resume their normal wandering functions. This means it may be used on a non hostile Walker Boss or Pirate Boss to lower their health to half a heart and then begin the fight.
These staffs can be found in high tier dungeon chests or mobs may sometimes drop them when you are in a dungeon.