I believe that in removing Zurin Arctus's heart, Tiber proceeded down the walking-way, along the mythic echo of Auriel removing the heart of Lorkhan/Shor before the dawn. Wulfharth is involved in that somehow.
Anyway, point is, the creation of the third empire mimic the creation of the world. The three's immurement in godhood was an essential step in laying the foundation.
The Underking arrives and is ambushed by Imperial guards. As he takes them on, Zurin Arctus uses a soulgem on him. With his last breath, the Underking's Heart roars a hole through the Battlemage's chest. In the end, everyone is dead, the Underking has reverted back to ash, and Tiber Septim strolls in to take the soulgem. When the Elder Council arrives, he tells them about the second attempt on his life, this time by his trusted battle mage, Zurin Arctus, who was attempting a coup. He has the dead guards celebrated as heroes, even the one who was blasted to ash... He warns Cyrodiil about the dangers within, but says he has a solution to the dangers without. The Mantella.
Someone always ends up getting maimed.