Scope the table nearest the ruined arch for a rather large number of arrows, a couple of Forsaken Bows, and another chest with still more goodies, and then head through the arch. The arch and the area right around it are full of ropes with bones hanging from them as an alarm system- one that the Briarheart will break coming down from here, and which shouldn't matter anyways, since he's the only one it would have alerted to begin with.
Check the table by the Briarheart's tent for some Soul Gems, then head up the stairs from here. Be forewarned- this is where things get really dangerous, but really interesting.
Make your way up all three staircases and you'll reach the source of the waterfall- a river coming down the mountain from another waterfall above it. There's an island with ruins on it in the midst of the river, with a set of stone bridges reaching out from it not only to both banks, but to the top of the waterfall. You can also see a ruined tower a bit past the far river bank, which is actually treated as a separate location.
The island in the middle of the river is the most significant thing here, and it is what you should check out first. The ruins contain several interesting objects and events packed into a remarkably small area.
First of all, there are two hagravens here. The semi-human creatures are engrossed in their work, making them easy to sneak up on- and if you can, you may want to watch and wait as they do. The two hagravens are in the middle of creating a Forsworn Briarheart, and are very difficult if not impossible to distract from their task without attacking them. This, unfortunately, will get both of them and the Briarheart on your case at the same time, giving you one potent and two overwhelming sources of offensive spells coming at you (mostly fireballs) from creatures that are willing and capable of healing themselves partway through the fight.
Your best bet for dealing with them all is to be a spellcaster, as they rarely use physical attacks, instead preferring to lob flame at you from the first place they reach where they can see you. You'll need a huge Magicka pool and high skill with Ward spells, but you can win this as an endurance fight.
Alternately, if you're a stealthy archer, you can climb up into spots they can't reach on the mountain and pelt them with arrows. Be careful about this, as there are many areas that you can't cross without jumping that they are perfectly capable of crossing, albeit at a slow walk. Make sure to put rocks in the way- it's possible to block fireballs with rocks that you can fire back over with a bow and arrow.
If neither of these is true of you, though, you'll need to bring a follower and be fairly high level to stand any chance of victory at this fight. On the upside, the rewards are plentiful: the Briarheart is very likely to drop a Briar Heart, a rare alchemical item. The hagravens themselves provide you with Hagraven Claws and Hagraven Feathers (more rare alchemicals). There is at least another Briar Heart as well as some other alchemical supplies on the stone table where the hagravens were working, and a chest chock-full of treasure waits against the ruin wall.
