RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Khazard Helmet
The Old School RuneScape Wiki also has an article on Khazard Helmet.

The Khazard helmet is a dark coloured medium helmet that is part of the Khazard armour set. Players can obtain Khazard helmets by searching the guardscupboard[sic] on the ground floor of the northeast most building of thee Fight Arena, only after they have started the Fight Arena quest. It is one of the weakest melee helms in the RuneScape Classic and has the same armour bonus as the Medium iron helmet.

Players must wear this helmet as well as the Khazard chainmail to enter the Khazard prisons, which are involved in the Fight Arena quest. This piece of armour cannot be made using the Smithing skill, is sold no where, and cannot be traded. Players can obtain multiple helmets by using the drop trick and searching the guardscupboard[sic] again.