I think Zurin Arctus is much more logical. The people of Tamriel have conflicting views on how it happened, and Tiber Septim as a ruler. It seems more practical, however, that there was a power struggle as to how the Numidium is to be used after its original purpose at the hands of Tiber Septim. I don't see how exactly Arctus would be killed in the process of creating the Mantella. Just how exactly does this process work?
Secondly, is there a consensus on whether or not the "Underking", as a physical entity, is the withered, undead body of Zurin/Wulfharth or an entirely separate thing? The idea of the Numidium's destruction and loss of the Mantella implies Zurin Arctus merely died. However, unless I'm misinterpreting this, when Wulfharth is allegedly soultrapped to form the Mantella, he was already undead? And it claims in The Arcturian Heresy that he [Wulfharth] reverts back into ashes following this.
Additionally, the story has this excerpt:
Who is this? Zurin? If so, doesn't that make the "Zurin is the Underking" theory somewhat true, as in that story, he personally destroyed the Numidium (and in turn, himself) following the conflict with Tiber Septim?
So confusing...anybody care to enlighten me on this?