...Jurgen Windcaller (or The Calm, as he is better known today), became converted to a pacifist creed that denounced use of the Voice for martial exploits. His philosophy prevailed, largely due to his unshakable mastery of the Voice -- his victory was sealed in a legendary confrontation, where The Calm is said to have "swallowed the Shouts" of seventeen Tongues of the militant school for three days until his opponents all lay exhausted (and then became his disciples).

So we know that the Word Walls hold tremendous power and grant words of power to those who know how to read them. We know Jurgen was so mighty that he withstood the roar of seventeen tongues without a problem and we know an Amulet of Kings, a Dragonborn, a Dragon or a soul gem can all hold souls with the need of deadric script.

Yet all of a sudden Jurgen needs to use daedric runes to bind a soul? It seems to belittle the power of the dragon language, which kind of goes against that which the game is telling us: Shouts are the TES version of apps - there's one for everything.

It's a neat theory and quite compelling due to the fact that Soul Trap is from the conjuration school which has obvious connections with Oblivion, but I think the runes spell out "Windcaller" simply to mark his grave. It's the horn which holds the soul, not the tomb. You have to "activate" it - use it - to get the soul. It'd have been nice disturbing if there was an animation for blowing Jurgen's Horn () but alas we must merely imagine the act. I tell a lie, it is the tomb.  So the act of receiving the soul is in response to prayer, reinforcing that it is Jurgen's burial place and shrine. You're beginning to sell me on the daedric script/binding theory.

I agree with the hypothesis that the whole place is a test for Dragonborn, though. The soul at the end is the distinction between a Tongue passing the test and a Dragonborn passing it. I wonder if the horn itself holds a clue. Is it identifiable as belonging to a particular creature?

 I wonder if the horn itself holds a clue. Is it identifiable as belonging to a particular creature?

Hmm. The horn looks more like a war horn. In ESO, war horn is an ultimate ability that gives health/magicka/stamina abilities. As for creatures--other than goats:

Dremora Lords have horns. Could be a connection with the Daedric script on the tomb.

In Oblivion, Minotaurs had horns which had alchemical properties.

It can't really be argued that Ustengrav is a test for the Dragonborn, but I don't like the idea of it being like a standardized test to put every Dragonborn who comes to the Greybeards through. The idea of the guy's tomb being a glorified obstacle course is somehow really disappointing, and I mentioned before, it suggests that all the draugr you kill eventually get back up; not necessarily a bad concept, but it certainly makes your efforts seem wasted.

Perhaps it's his old war horn?

 it suggests that all the draugr you kill eventually get back up; not necessarily a bad concept, but it certainly makes your efforts seem wasted.

Have you stumbled across the true reason why the draugr are restless? That, in most cases, you have to venture into a dangerous tomb to find a word wall and that you must learn to "temper your abilities with wisdom" as Arngeir puts it?

This idea brings us to the meta-game which underpins all the TES games, that it all exists for the Hero to sort out and that the reason the draugr, the dragons and all that is going on is because the Hero has appeared. As Jagar Tharn says:

Each event is preceded by Prophecy. But without the hero, there is no Event.

A true agent of change for without the Hero, nothing happens.

I like to see the game(s) as a Rube Goldberg machine, with some elements of chance and divergence, and the Hero is the ball that runs the course. Without the ball, nothing happens, but the machine is still sitting, waiting and ready, and these discussions are us, looking at the workings of the machine.

And the machine is laughing at us.

It's not like there are tons of people jumping at the chance to be Dragonborn. You don't get anything for it and you'd have to pass the first test which is being able to shout which takes years of practice or being Dragonborn. They pretty much know you are one and honestly I don't understand the Ustengrav test unless it is simply to pray at Jurgen's tomb and develop some connection to him. Maybe the Greybeards think it'll keep you on the path to following the Way of the Voice?

The games focus upon prophecies within the Elder Scrolls. The player acts as the Observer, witnessing events and solidifying history.

...You mean we're supposed to be writing all this down? Sh**.

To receive a soul you must a) have the horn in place b) be the Dragonborn. One doesn't work without the other. Thus, I think, the runes are connected to the horn. That's why they spell "Windcaller", for the horn to act as a catalyst for the soul release. I think it's his horn and that's why runes release the soul when you return it. 

With Nord respect for their ancestors and burial sites (as we see in Hillgrund's Tomb quest), I doubt that the Greybeards would've used the resting place of the founder of their order as a glorified obstacle course. Consider, that they give you the task to get an item from the barrow, not really respectful.