The dwarven helmet is obtained as a reward for completing the quest Grim Tales.
If lost, a replacement can be obtained by talking to Rupert the beard beside Miazrqa’s tower, where he was kept prisoner during Grim Tales (between Goblin Village and Ice Mountain), for a fee of 60,000 coins. A Defence level of 50, as well as completion of the quest Grim Tales, is required in order to wear the dwarven helmet.
The dwarven helmet has similar stats to a rune full helm, though it is worth noting that it has a +6 Crush attack bonus, making it the Best in slot for crush attack bonus.
Wearing the helmet inside the Dwarven mines is a hard requirement for the Falador Diary.