I suppose I did forget one thing that happened on the way to Ustengrav.
Looks like there was a bit of commotion at the entrance to Ustengrav.
Inside there are more necromancers. Looks like they finished off the bandits there, and of they don't like me too much either. Not that it matters to me; bandits would have given me just as much trouble. I make short work of them. Eventually I reach the front group of necromancers - they are stopped by draugr, and eventually are finished off. Now I get to take on the dead guys. They aren't too much trouble, really. Without much delay, I get to the chamber where the Horn is.
Or where it should be, at least.
Looks like someone got there before I did. Someone that knew I was coming for the Horn. They left a note in place of the Horn, saying to go rent the attic room at the inn in Riverwood. Funny, I don't remember there being an upstairs room there...smells like trouble. At any rate, that's where I have to go.
I leave the crypt, and head to nearby Morthal to stock up/sell loot. Then back to Riverwood.