It all comes back to Hermaeus!  Look at him dance, he knows he's got the moves.

The question is, did Jurgen know he was siding with Old Hermy? If I wanted a devout follower of the gods and strongest tongue in Skyrim to remain silent for the rest of his life, I might not approach him as I am, rather I might assume a disguise. Put on a hat or something.

The statues are dragons but do we see them anywhere else or is that particular style unique to Ustengrav?

The dragons symbolically rise when you enter the chamber, ushering in the new "soulcaller" aka Dragonborn.

This I like! The Dragonborn needs to know Whirlwind Sprint in order to get that far. The Horn is placed in a location that only Tongue or Dragonborn could get to (or Delphine if you cheat) and could be a literal "soulcaller".  Yet we're supposed to believe Jurgen wasn't Dragonborn? I don't buy it.

I know this is off topic, but if the Greybeards use retrieving the horn as a test to see if you are the Dragonborn then would they have used it to test other's such as Tiber Septim and if so did they then go to the trouble of placing back in the tomb?

Yeah, we can assume by Delphine's "Greybeards are nothing if not predictable" speech that this is a tradition that won't die. Can you imagine the selection process they must go through, like picking straws to see who has to make that journey after the Dovahkiin returns with the horn.

Or maybe that's why they use people like Klimmek and the true reason for Klimmek's Sack? Paired with Jurgen's Horn it is a powerful phallic symbol of Greybeard might which protects whatever unlucky bastard has to return to Ustengrav?

That would seem to suggest that Jurgen kept his ability secret. Why wouldn't Arngeir mention that he was a Dragonborn? The dragons also weren't extinct during his time (not all of which was peaceful) so why would this war leader not use the gift to continue whittling them away? It seems unlikely to me that he was Dragonborn.

Here's a theory, pulled right out of the air: Jurgan the War Leader served the dragons(possibly just in a religious sense) until being defeated at Red Mountain. Seven years of meditation later, he has changed stance, and Jurgen the Calm serves only the divines.

I seriously doubt this, unless we also accept that draugr never truly die. If they doo all eventually get back up and go back to their sarcophagi, then maybe. Otherwise, there would be none left. It makes sense as a trial/tradition, but the implementation wouldn't work. Even Delphine apparently had to kill draugr to get at the Horn. More likely to me is that this fated "Last Dragonborn" came attached with some line of prophecy about retrieving/restoring the Horn.

Perhaps the Draugr in his tomb were his soldiers when he was a leader. As for the Dragon soul you gain, maybe he's transferring knowledge.

This actually sort of fits the notion that Jurgen 'the War Leader' may have worshiped the dragons in his "blasphemous misuse of the Voice", and during his lifetime the tomb was built for him(presumably with warriors being buried there through the years). But by the time he actually died, he no longer followed the ways of the dragons. And they weren't going to waste a perfectly good tomb.

I don't think it's too unusual that all the tombs in Skyrim are draugr infested, though. Most of them contain word walls, sure signs that those buried in them worshiped the dragons. We also see that modern Nords don't go to the trouble of makes such tombs anymore. A change of beliefs? It fits the pattern.

Placed to guard the horn...by Mora, then? Maybe you should make a post fully explaining the Mora theory, for those unfamiliar XD

That would be interesting, but almost worse for his case. It would suggest to me that he was using his soldiers just as the Dragon Priests do their draugr, to keep himself alive. His presence in Sovngarde runs contrary to this, though. None of those folks appear to be able to influence mortal Nirn.

No, I meant, his soldiers were interred with their general. Not so he could feed off them, but so they could join/protect him in Svengarde, like win the Pharaohs of Eygpt.