Ok, just recently got all dragon priests masks and I read on the Wiki that once I get Konahrik's mask I should have the ability to summon dragon priests. It's not in my conjuration spells, I can't buy it anywhere, is there a certain skill level that I have to be in conjuration that lets me obtain it?
The way it works is you wear the mask and when you're low on health the mask automatically activates one of two effects. The first one knocks back nearby enemies, activates flame cloak, and heals you and your allies. The other one is conjure Dragon Priest. It's not really an additional spell, but more of a safety net.
Once you get Konahrik's mask, while wearing it, when your health drops below a certain threshold it has a chance to activate one of two effects, one is that it summons a dragon priest (Supposedly Konahrik himself) to protect you and assist you with attacking enemies he can also conjure his own atronachs, and second the mask emmits a shockwave not unlike unrelenting force which knocks enemies within range backwards and deals minor damage, it then gives you a moderately damaging flame cloak which burns enemies for every second they are in melee range and heals you and any allies around you (Very much like the Avoid Death Restoration perk but without the daily limit) These effects may not activate all of the time but it has a high chance to do so.