Quoted
Dragon Priests were once the primary servants of the ancient dragons who used to be worshipped in Skyrim and would rule over countless armies of men in their god's name, the men who are now the draugr of those ruins rest with them until disturbed... They are found residing in coffins in areas of prominent Dragon worship, particularly http://www.uesp.net/wiki/Skyrim:Nordic_Ruins. If their resting place is disturbed by trespassers, their coffin will fly open and they will awaken and proceed to attack intruders with magical attacks and summoned minions. Dragon Priests were great wizards from all races that happened to become liches, undead and immortal necromancers.
In combat, the eight masked Dragon Priests are incredibly dangerous and can kill any level player very quickly if not fought properly. They will constantly move away from the player character at a high speed making it very difficult to land any melee strikes. While moving away they will use their staves, all of which have different powers.
Unnamed dragon priests can also be summoned by the player using conjuration magic, but these versions are significantly less powerful than their boss level counterparts.
Eight Dragon Priests are named bosses, each wearing a http://www.uesp.net/wiki/Skyrim:Dragon_Priest_Mask with a powerful effect. Each of the eight is challenging in their own way, and show many traits similar to those of Dragonborn, including mastery of the language of dragons and use of some shouts.
Furthermore, there is a possibility that some named bosses, such as http://www.uesp.net/wiki/Skyrim:Curalmil, will appear as Dragon Priests. However, this is merely a leveled boss (bosses like Curalmil or Red Eagle can be Draugr Deathlords or lower, if the player is of a low level). These types of leveled bosses do not drop any masks, and are not part of the relevant quest to collect them. Also, random unnamed Dragon Priests may appear, after the player has exceeded the Draugr Deathlords.
Mask Only: http://www.uesp.net/wiki/Skyrim:Konahrik
Dragon Priests: http://www.uesp.net/wiki/Skyrim:Hevnoraak_(creature) | http://www.uesp.net/wiki/Skyrim:Krosis_(creature) | http://www.uesp.net/wiki/Skyrim:Morokei_(creature) | http://www.uesp.net/wiki/Skyrim:Nahkriin_(creature) | http://www.uesp.net/wiki/Skyrim:Otar_the_Mad | http://www.uesp.net/wiki/Skyrim:Rahgot_(creature) | http://www.uesp.net/wiki/Skyrim:Vokun_(creature) | http://www.uesp.net/wiki/Skyrim:Volsung_(creature)
Why is Konahrik a mask only????