I've mentioned thu'um using draugrs elsewhere, but it's one reason why I'm a little skeptical of the tablet story. I don't know if it's oversight on Bethesda's part, or meant to conflict with it.. but the tablets say that Paarth taught humans the Thu'um. So who are the dragon priests and draugrs? They served dragons, subjugated their fellow mortals, and could use the thu'um. Apparently without Paarthurnax's help. if the thu'um was being used in service of dragons, then it conflicts with the tablet story. There, it was apparently the turning tide that won the dragon war for men, while you have these other dudes who enslaved men.
It's also a little odd that you only fnd these guys by word walls. And the Greybeards have a connection to word walls too. Is their order really a reformed order of dragon priests? That doesn't make them bad or anything, but I'm just wondering.
This explains everything, a book that's actually in the game. http://elderscrolls.wikia.com/wiki/The_Dragon_War
Foremost among all animals was the dragon. In the ancient nordic tongue it was drah-gkon. Occasionally the term dov-rha is used, but the language or derivation of that is not known. Using either name was forbidden to all except the dragon priests. Grand temples were built to honor the dragons and appease them. Many of them survive today as ancient ruins haunted by draugr and undead dragon priests.
Dragons, being dragons, embraced their role as god-kings over men. After all, were they not fashioned in Akatosh's own image? Were they not superior in every way to the hordes of small, soft creatures that worshipped them? For dragons, power equals truth. They had the power, so therefore it must be truth. Dragons granted small amounts of power to the dragon priests in exchange for absolute obedience. In turn, the dragon priests ruled men as equals to the kings. Dragons, of course, could not be bothered with actually ruling.
In Atmora, where Ysgramor and his people came from, the dragon priests demanded tribute and set down laws and codes of living that kept peace between dragons and men. In Tamriel, they were not nearly as benevolent. It's unclear if this was due to an ambitious dragon priest, or a particular dragon, or a series of weak kings. Whatever the cause, the dragon priests began to rule with an iron fist, making virtual slaves of the rest of the population.
When the populace rebelled, the dragon priests retaliated. When the dragon priests could not collect the tribute or control the masses, the dragons' response was swift and brutal. So it was the Dragon War began.
At first, men died by the thousands. The ancient texts reveal that a few dragons took the side of men. Why they did this is not known. The priests of the Nine Divines claim it was Akatosh himself that intervened. From these dragons men learned magics to use against dragons. The tide began to turn and dragons began to die too.
/e So basically dragon priests aren't men, or just warped versions of them. And parth wasn't the first one to teach the thu'um, he just taught the nord hero's who brought down alduin.