Just wondering what the deal is with these guys really,. Very minor spoiler going into slightly more detail below

There's a sort of 'ritual' involved where a member of the Forsworn allows a Hagraven to cut out his human heart and replace it with a briarheart. A bit creepy, really. :/

also if you are a thief,
(dont know how to do spoiler tag so,,, slight SPOILER)

you can pickpocket their briarhearts and they will die, like ripping out their heart

I know that, that's not what's bothering me, thanks for the help though:). I just can't figure out what the briarheart itself (as in the actual object) is, and how it could replace a human heart, and why someone might want it done

That's cool!

Well, I assume, the hagravens' replaces a forsworn's heart with something called a Briarheart, it makes them more an effective killing machine though I'm not sure if they're concious or possessed.

Oh, sorry.
All I know is that it grants them new powers, and makes them A LOT more difficult for us to fight!
I'd check the UESP article on Briarhearts!

Oh you want to know about the Briarheart? I'm not sure what it is, you might want to look it up on the UESP.

From Uesp, on the actual ingredient:
While their exact nature is unknown, due to its natural loot location from http://www.uesp.net/wiki/Skyrim:Forsworn_Briarheart it seems they really do serve as actual hearts; from the look of events at Lost Valley Redoubt, dead Forsworn are resurrected in a ceremony by Hagravens, replacing their human hearts with these items. Interestingly enough, pickpocketing the Briar Heart from a Forsworn Briarheart will instantly kill the target.

So, pretty much nothing new:/ I just would have thought it would be a cool element of the lore to explore

On the guys themselves:

"They have been created by a http://www.uesp.net/wiki/Skyrim:Hagraven from a Forsworn corpse. As such, Briarhearts are classed as undead creatures and vulnerable to undead-specific spells. However, they can be detected with http://www.uesp.net/wiki/Skyrim:Detect_Life, rather than http://www.uesp.net/wiki/Skyrim:Detect_Dead, as would be expected. This might be because the Briar Heart itself is living, but it might just be an oversight. "

So it's not an operation, they're already dead. Cool I guess

What is the actual Briarheart? it look like some kind of plant. Made me feel abit sick when I first came across it .

That's what I've been trying to find out haha, what it actually is and why it reanimates dead bodies. UESP just said "their exact nature is unknown"
Is there some kind of in game book on it or something?

Forsowrn are realy messed up if they let hag ravens do all this twisted stuff to them.

Yep it just says its an ingredient but doesn't actually say what it is or were it comes from dose it grow on some tree or what or the hag ravens make them?

Yeah, I'd actually like them if they didn't do all that creepy stuff, I mean, I sympathise for them what with getting kicked out of Markarth and all, but seriously, they're not gonna win popular support by being chummy with hagravens.

[censored] briarhearts are impossible on master one even did a finisher on my summoned dremora lord it stabed him through the stomach and lifting it up into the air.I really wish they can be reanimated.

Yeah, it said on UESP they level all the way up so they'll stay hard. On my current expert playthrough they're a challenge to say the least.

I suspect their nature magics have something to do with Kyne.
I found a shrine to Kyne in one of their towers, surrounded with offerings.

Seems fairly likely.
I think they're one of the most interesting factions in the game, pity that hard info on them is so infrequent and ambiguous.

But that's exactly what makes them so interesting. A lot of TES lore is ambiguous and vague. It makes it more free and open to interpretation, which is why TES is so unique.

Anyways, I would say that it's a ritual to Sheor. Taking out someones heart is very fitting for an aspect of Lorkhan, is it not?