I honestly am at a loss at what happened here.

Dont have a screenshot, but... While fighting Miraak at the Summit, at one point, I summoned an Ash Guardian. It didnt do much. BUT. At one point after Miraak ate a dragon, suddenly BAM. This... Thing appears. I cant even describe it very well but it sort of resembled a Mistman from Dragonborn. Somewhat armored torso, Helmet of some description... I couldnt sqq it very well, since Miraak is shouting up a storm trying to kill this thing and Ash Guardian blowing smoke everywhere. All I could see, for SURE, is that it had a 2 handed Draugr War Axe, and the instant it appeared, it shouted, cloaking itself in the Dragon Aspect Shout.

Now. I do have mods, but... Nothing like that. Definitely nothing that can do a Dragon Aspect Shout. I dont have the 9th Mask from collecting the other 8. I didnt summon anything except for an Ash Guardian. What the hell did I just see?

The Ancient Dragonborn. When you use Dragon Aspect and your health drops below 50%, he's automatically summoned. He's quite powerful and he'll use several shouts in battle.

That is very cool.