If you want to go down the Slayer Upgrade tree, then yes, you will eventually need this mask because it does allow you to make the powerful slayer helmet. Additionally, the extra damage and strength against creatures you are on a slayer task for can be extremely helpful. Despite the fact that you have no control over it, the extra effect of the passive defense drain can also be very helpful whenever you are fighting tougher creatures. That being said, you don’t need it if you don’t want it. There are plenty of other pieces of armor in the world and you don’t need them all. Pick what is best for your character and for your enjoyment of the game.