Rurelion is an Altmer who has been possessed by an undead Maormer wizard named Uldor at the Temple of Mourning Springs during the quest Tears of the Two Moons. His assistant is Gathwen, who will run for help to save him from his possessor.
Rurelion is being controlled by Uldor, and is being used to summon more undead. The Vestige must defeat numerous skeletons to stop the ritual. After this, Rurelion begins fighting off Uldor's control, and they rush off. Eventually, Rurelion forces Uldor out. The Vestige has the option to either seal Rurelion or Gathwen inside the tomb. If Gathwen is chosen, Rurelion will get the Mourning Stone. He will swear to free Gathwen from Uldor eventually.
If Rurelion is chosen to live, then he will offer to fight alongside the Vestige against the Sea Vipers.