But the dragon born DLC has given us more information on this front. Master Neloth, the Telvanni wizard, says that the Briarhearts do what they do because it gives them a direct connection to "the forces of nature." He also says that he wants to study the Briarhearts to so he can tap into the power of Red Mountain. Moreover, we now have Ash Spawn which can only be summoned and maintained using a heart stone.
This got me thinking. I've seen people mention before that Dagoth Ur was merely a puppet of Lorkhan, that he was executing his will on Nirn. Now we have the fact that even with Dagoth Ur gone, the ash monsters seem to be doing just fine (provided they have Heart Stones.) Talvas Fathryon says the Heart Stones are fragments from the eruption of Red Mountian that happened 200 years ago (Red Year.)
When you finish the Heart Stone quest, he casts a spell on you that is supposed to "tie you to the realm of the Aedra" and make you stronger and more powerful. (further evidence of Aedra in Aetherius btw.)
I'm thinking that when the Heart was destroyed, Lorkhan's spirit was unbound. Now more difuse, it is affecting the world again.
Any more insights to be drawn from this info?