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

A Large rune helmet is the second best helmet in RuneScape Classic. It requires level 40 defense to wear and can be obtained as a drop from greater demons and black demons. They can also be smithed from two runite bars at level 92 smithing, giving 150 experience.

Dropped by

Monster Combat level Quantity Rarity
Black Demon 156, 175 1 3Uncommon
Greater Demon 87 1 4Rare