A bit Of Lore:
"Zurin Arctus was the Imperial Battlemage of Tiber Septim. He was responsible for resurrecting the Numidium, a powerful golem that helped Tiber Septim to conquer Tamriel. Upon the destruction of the Numidium, Arctus was either killed by or transformed into the Underking, according to different accounts.
The Numidium was powered by the Mantella, a powerful gem infused with the heart (life force) of a person. According to one account, Zurin Arctus sacrificed his own heart to the Mantella. After using the Numidium to conquer Tamriel, Zurin and Tiber Septim disagreed over how to continue using the Numidium. When Zurin tried to reclaim his heart, it was blown into Aetherius, destroying both the Numidium and Zurin. Zurin was left in a semi-slumber in a subterranean vault, waiting for an opportunity to reclaim his heart. In his undead shape, Zurin Arctus became known as the Underking, and from time to time he sent out his forces to find the Mantella throughout Tamriel. During the events of The Warp in the West, the Underking was able to retrieve his heart, and finally passed into true death.
However, in another account (see The Arcturian Heresy), it is written that the Underking was, in fact, King Wulfharth of Skyrim, a powerful undead being from the First Era. He was tricked by Tiber Septim and Zurin Arctus, and his soul was trapped into the Mantella to fuel the Numidium. Zurin Arctus himself was killed in the process. To prevent some political unrest, Tiber Septim stated that Zurin Arctus tried to assassinate him.
It is rumored that the Underking assisted Almalexia in destroying Kamal's king Ada'Soom Dir-Kamal at Red Mountain, when he attempted to conquer Morrowind. It is also speculated in the West that Zurin Arctus brokered the agreement between Tiber Septim and Vivec over the fate of Morrowind, and enchanted the famous Dragonbone cuirass.
He is mentioned by one of four former Blades, Rielus (who was sent to the catacombs of Sancre Tor by Tiber Septim to discover the source of the evil infestation), as his captor who made him haunt the catacombs after death, and the one who sealed off the room containing Septim's armor and populated the site with undead minions. Mannimarco, the King of Worms, also claims that he had a long, ineffective conflict with the Underking."
"But after nine months that mound of mud became as a small mountain, and there were whispers among the shepherds and bulls. A small community of believers gathered around that growing hill during the days of its first churning, and they were the first to name it the Golden Hill, Sancre Tor. And it was the shepherdess Sed-Yenna who dared climb the hill when she heard his first cry, and at its peak she saw what it had yielded, an infant she named Reman, which is 'Light of Man.'" — Remanada
Sancre Tor was a fortified city once conquered by Tiber Septim that later fell into ruins. The mythical birthplace and final resting place of Reman Cyrodiil,it is located in the Jerall Mountains in Colovia, north of the city of Chorrol.In 2E 852, the city was captured by invading forces from High Rock and Skyrim. General Talos, who would later become Tiber Septim, recaptured the city that winter,however as the city was in a strategically inconvenient position it was then abandoned. The old fortress of Sancre Tor was later corrupted by the Underking, prompting it to be sealed shut by the Grandmaster of the Blades in the 36th Year of the Reign of Tiber Septim.In the years after Tiber Septim's reign it was gradually forgotten, decaying into a haunted ruin. The Champion of Cyrodiil would visit the ruins in the year 3E 433 to recover the armor of Tiber Septim."