What do you guys think? Based on what we know which Daedra do the Forsworn Worship
I always like to pretend they worship the Breton god Sheor, from varieties of faith. Since it's such a breto-nordic deity I can imagine him being worshipped in both eastern Highrock and western Skyrim.
http://www.gamesas.com/topic/1431560-the-old-gods/?hl=%2Bforsworn#entry21958741
Bottom line... we actually don't know.
Neloth is experimenting with Briar Hearts... it seems that their life force is being bound to the new heart. He plans to do that with Heart Stones instead of... briars.