Slayer helmet

It is quite useful because it gives a larger defense bonus than any of the items separately. There is a 15% Attack and Strength bonus from the Black mask that only appears during a Slayer assignment. This will give you a boost in the max hit you can do against a monster. You can acquire the knowledge on how to make this item from any of the Slayer masters.

And as per usual, the items that go into the actual helmet continue to work against monsters that require say earmuffs or a face mask to kill.

These include:
Banshee
Dust Devil
Aberrant Spectre
Wall Beast

After you've finished Smoking Kills you can gain points for different Slayer assignments given to you. Once you reach 400 points total you can use them to learn how to make the Slayer Helmet.

It takes level 55 Crafting to make the helmet after gaining points to learn how to make it. Also it takes 10 Defense to wear it.