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

A Medium black Helmet is the fourth weakest medium helmet in RuneScape Classic. Players need level 10 defense to wear it. As with all Black armour, players cannot make this item using the Smithing skill. A player can dig up one in the Digsite or obtain one as a drop from killing Dark Warriors in the wilderness.

Dropping monsters

Monster Combat level Quantity Rarity
Dark Warrior 21 1 3Uncommon