So, some sort of undead?
But what strikes me as curious is that Briarhearts have their hearts crudely cut from their chests -- which has some rather strong resonances -- and replaced with the briarheart.
To be clear, this seems an echo of Lorkhan, and also of Pelinal Whitestrake, who had a permanent wound in his chest that showed a red diamond where his heart should be. There may be a hint here, too, of an association with dragons, or at least the dragon cult.
Is there more we know of the Briarhearts?

its both permanent and doesn't turn its victim into a slobbering idiot .