The dragon full helm is a helmet that requires 60 Defence to equip. The dragon full helm gives better defence bonuses than its medium helm counterpart. Due to its high price, and lack of prayer or strength bonuses, it is more often used as a sign of wealth, rather than in combat.
A dragon full helm can be combined with a dragon full helm ornament kit to make a gold-trimmed dragon full helm (g). This cosmetic enhancement won't change the stats in any way.
Players can obtain a dragon full helm as a very rare drop from mithril dragons in the Ancient Cavern. The chances of obtaining one in this manner is 1/32,768.
Alternatively, players can receive one from a barbarian spirit after lighting a pyre ship with chewed bones. The chance of this occurring is 1/256. However, chewed bones themselves can only be obtained from mithril dragons as an uncommon drop, with a drop rate of 3/128. The effective drop rate via chewed bones is 3/32,768 (or ~1/10,923).
Taking both the drop from chewed bones and the direct drop from the dragon into account, the effective drop rate is ~1/8,192 per dragon.
A player wearing a dragon full helm.
For an exhaustive list of all known sources for this item, see here