So, I got the Red Eagle sword, got the next misc quest to unlock the mistery, then it just ends...
I went to the cave, placed the sword in it's pedestal and I blieve the game sayed the quest was completed at this point. Then I just went and killed without trouble (at lvl 36) an already dead hero and nothing more.
This feels like I'm missing something, what's the big mistery I uncovered and what have I acomplished by that. It was no noticable unique loot there...what I'm missing?

That was it. It was a small quest, that you'd come across randomly without being directed to it

There should of been some Unique Loot in the chest?

I did it that quest I hate that the quest repeat it self when nothing happen if you do it again.

Red Eagles Blade becomes Red Eagles Bane, an effective weapon against the undead, and you get a nice Claymore off of his corpse.

Didn't nottice the transformation. I notticed the two handed sword on the corpse but didn't think is something that good and definitly not unique...but I'm a dual wielder.

I still can't figure out why a Briar heart has his bane.

You did take the sword back out of the pedestal when you were done?
I made that mistake too. When I did go back and get it, turns out it was pretty decent and I used it right up until I got Dawnbreaker.

the secret is in who they "worship" I think... at least that's what I took from it because I'm a newb to the lore of the game entirely and pay attention to all this stuff as a way to learn about it.

the blade upgrades if you take it back out of the pedestal... and otherwise it's sort of a lootable museum tour.

all the complaints about how the old guys might just be a new form of Dragon Cultists and... zomg... once you know this chain then the clothes on the forsworn really make sense.

If I'd been Ulfric I wouldn't have settled for just their unrepentant leaders when I won the Reach back. I wonder if he'd think that in hindsight if he were "real"? Knowing he'd go to jail anyway?

You could always read the book again and the quest will start over.