At one point, Uldor was once a powerful mage, who could control legions of undead, being powerful enough to transcend his own death. He was eventually locked away in the Temple of the Mourning Springs on Khenarthi's Roost by the Maormer. They gave the local Khajiit there the Mourning Stone to keep it sealed.[source?]
In 2E 582, Ealcil, Gathwen, and Rurelion went on an expedition into the temple. Ealcil removed the Mourning Stone from it for research, unaware of its purpose. Immediately after, the party was attacked by undead, controlled by Uldor. Ealcil successfully escaped, but the rest of the group remained, and one of them was ultimately taken over by the Maormer wizard.
Uldor is shown to be arrogant and greedy, claiming things that he does not possess to be his. He states that the Vestige's body will soon be under his control the first time that he finds them. He desires Eagle's Strand as well, seeing it as an opportunity to increase his power with more potential puppets for his ultimate conquest. He is also possessive of his trophies, stopping the spirits whose bodies he has from escaping, as well as wanting to keep Rurelion under his influence.
Since the removal of the Mourning Stone, Rurelion has gone missing, causing Gathwen to recruit the Vestige to join her in finding her master. Uldor will appear as a skeleton at the front gate, warning the two of their eventual fate—to be his minions—as well as explaining that Rurelion is his. He wards the catacombs of the temple twice, and places sentries outside to protect his ritual.
Once the catacombs are entered, and the puzzle complete, the pair will find Rurelion speaking strangely; Uldor soon reveals that he is possessing Gathwen's master, explaining how he was imprisoned at the temple by the Mourning Stone until it was removed. He refuses to take over the Vestige, calling the body poisonous and soiled attire. He begins the ritual to create an undead army to wipe out Eagle's Strand, but is thwarted when the Vestige and Gathwen dispel it. Uldor angrily states that his plan is not over, but Rurelion takes the moment of weakness to fight for control, running into the Great Hall.
In the next room, the Vestige will find Rurelion free of Uldor's control for the moment, having forced him back into the tomb beyond the hall temporarily. After the traps are disarmed, Gathwen will arrive to hear that Rurelion plans to enter the tomb as a distraction while the Vestige places the Tears of the Two Moons on the pedestals, trapping Uldor inside. She will volunteer to be the distraction instead, forcing the Vestige to choose between her or Rurelion. Whoever is chosen will accompany them inside, while the other retrieves the Mourning Stone.
As the Vestige and whoever is chosen enter the tomb, Uldor will fall for the plan, taking control of either Rurelion or Gathwen. He will then send lightning strikes at the Vestige as he cackles. Once the Vestige places the stones on the pedestals, he will be trapped in the tomb with either Rurelion or Gathwen.
When fighting Uldor, the best way is to activate the pillars as quickly as possible, staying mobile unless one is capable of absorbing a lot of damage. Uldor is only able to be defeated if all four stones as placed, and, therefore, cannot be attacked. If the Vestige is about to die, leave the room. Uldor cannot attack with his spells outside of the tomb.
Uldor can control any corpse or skeleton that he can access. He uses numerous of them, including Skeletal Warriors, Skeletal Archers, Skeletal Cryomancers, and Revenants to guard the temple from the Vestige. He also summons more to take over Eagle's Strand, although the ritual is interrupted.
Uldor can also send bolts of lightning at his enemies, dealing ranged damage from above. Although they take a bit to charge, and more to fire, they can be deadly to a stationary enemy.