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?