I have a pretty good feeling That practically all of the events of the main Elder Scrolls series deals, in some part, with Lorkhan trying to "reassemble" himself and/or enacting his little plan. I know he's trying to get his heart back because he's done that through his many "manifestations", such as the Underkings and perhaps Dagoth Ur in a warped sense, but...why??
What could he possibly want to do after he does that? I have multiple theories but I don't know which one to trust. May sound like a dumb question but I figured I'd just ask anyway.
Edit:
hell, what the hell was he trying to do in teh first place? I realize it was an attempt at CHIM, like Vivec states, but...how??