Primal full helm is a tier 11 full helm that is only available while Dungeoneering. It requires level 99 Defence to be worn. Primal full helm is the best possible full helm to obtain in Dungeoneering.
As with all Primal equipment, this cannot be made using the Smithingskill.
The only way this item may be obtained is as a drop from the Skeletal Horde boss. The helm used to be a very rare item to find. This was owing to the fact that the drop rate of Primal equipment is affected by the combat level of the boss, and the highest combat level of the horde is very low for that of a boss, thus resulting in a very rare chance for a Primal full helm drop. However, on 10 April 2013, the rate at which the Primal full helm is awarded was brought in line with the other primal items  , thus making it much easier to obtain. The rate is now similar to the rate of getting any Tier 11 item from the other Dungeoneering bosses.