Sul was the husband of Ilzheven, a female Dunmer who was one of those selected to power Vuhon's Ingenium. The device served to keep the Ministry of Truth aloft, after Vivec City disappeared during the events of the Oblivion Crisis, and was powered by souls. Sul attempted to free his wife, but damaged it when Vuhon attempted to stop him. This caused the Ministry to resume its course and crash into Vivec City.
Sul and Vuhon were propelled into Oblivion, specifically the realm of Clavicus Vile. There, they encountered a being called Umbra, who was technically part of Vile, which threw the Umbra Sword into Mundus through the rift the Ingenium created. Vuhon made a deal with Umbra for his life: in exchange, he would build a new Ingenium to allow Umbra to escape Vile's realm. Sul was kept prisoner and tortured for years, but grew bored and forgot about him. Sul escaped into Azura's realm, where she almost drove him mad with visions. 
Sul was able to escape Oblivion after decades and return to Mundus. Azura still sent him visions, though of a more helpful nature, so he could get his revenge on Vuhon. She guided him to Attrebus Mede, eventually finding him in Elsweyr and rescuing him from kidnappers. Sul made Attrebus realize that his entire "legend" had been a fabrication designed to make him a hero of the people. Attrebus decided to help Sul bring Umbriel down.
The two eventually encountered and saved a group of Khajiit from corrupt Imperial soldiers, who then pledged themselves to them. The group traveled back into Cyrodiil, to the ruins of an Oblivion Gate outside of Water's Edge. Sul planned to travel to the ruins of Vivec City using Oblivion as a shortcut of sorts. They travelled through several realms before being stopped in Hircine's realm, where they were attacked by Hircine's minions. The Khajiit sacrificed themselves to buy Sul and Attrebus time to reach the exit.
The pair emerged back in Mundus at the ruins of Vivec City. Sul knew Umbriel was heading there, as Vuhon knew the Umbra sword was there and that it could be used to destroy him. However, the sword was gone. Sul used his powers to contact the ghost of his dead wife, Ilzheven. She informed him that a group of Dunmer had been there and taken the sword and taken it towards the Sea of Ghosts.
Attrebus then warned Sul that Umbriel had arrived, and Sul's undead army was quickly upon them. The pair attempted to fight their way back to their entry point to Oblivion, but they were swiftly overpowered by the undead and brought before Vuhon in Umbriel. Vuhon only need the White-Gold Tower to emancipate Umbriel from Oblivion, not the people in the city, so he offered Attrebus the opportunity to be a hero by leading the people out of the Imperial City before he arrived. He refused, and Sul was able to bring himself and Attrebus back into Oblivion before Vuhon could stop them.
The duo appeared in Namira's realm, where they were attacked by an unknown Daedra which severly wounded Attrebus. Sul was able to bring them into Malacath's realm, where the Daedra Prince healed Attrebus and sent the two on to Solstheim where the Umbra sword was located, as he approved of Sul's lust for vengeance. 
Once in Solstheim, the two discovered that the sword was being held in the keep of a Dunmer lord named Hleryn Sathil, whose son Elhul had found it on an expedition to the ruins of Vivec City. However, when Elhul touched the sword he went mad, slaughtering many in the keep before being subdued and locked in a cave beneath Hleryn's keep. Somehow the sword sustained Elhul's life who, even without food or water for years, was incredibly strong. 
Sul and Attrebus were able to kill Elhul and take the sword, however Hleryn's wife Nirai refused to release them from the cave unless they left the sword, afraid it would cause harm to others as it has her family. Before she could do anything to harm them, Sul realized that as an artifact created from Clavicus Vile himself, it could be used to bring Attrebus and himslef into Vile's realm. The two escaped Solstheim that way. 
In Vile's realm, the Daedra Prince struck a bargain with the Attrebus and Sul: in return for re-uniting Umbra and the sword, Vile would take Umbriel from Mundus back into his realm, seemingly no strings attached. He then sent the two as close to Umbriel as possible, his shrine west of the Imperial City.
Vile made them appear several feet off the ground, and Sul was knocked unconcious in the fall. Attrebus was able to find a boat and row the two across Lake Rumare, but they were captured by Vuhon's undead and brought before his ally who had helped bring Umbriel into Mundus, Minister Hierem. Hierem held them in a hidden room in White-Gold Tower and wanted to know why Vuhon feared Sul and used a strange torture to get Attrebus to talk: he used magic to make him feel intense pleasure and then riping it away from him, nearly driving him mad. Sul finally told Hierem about the sword.
As Hierem met with Vuhon on Umbriel, Sul and Attrebus were rescued by a Penitus Oculatus Inspector named Colin and an Imperial spy named Lettine Atrese. Sul and Attrebus were able to use the portal Hierem used to travel to Umbriel and were brought directly to Vuhon. Before he could move, Sul drew Umbra and drove it into his chest. This had no effect, however Annaïg Hoïnart had poisoned the Hist-like trees Fringe Gyre moments before, greatly weakening Vuhon and allowing Sul and Attrebus to escape.
After meeting up with Mere-Glim and Annaïg, they realized that Vuhon had stored his soul inside the Ingenium along with the being of Umbra, and only by stabbing it with the Umbra sword could the crisis be ended.  Fighting their way to the Ingenium, Attrebus used the sword to stab it. However, Clavicus Vile possessed him at that moment and tried to take his soul. Sul confronted Vile, who told him he was free to go if he chose. Sul merely stated it was supposed to be himself who died, then attacked Vile. The Daedra Prince stabbed Sul and claimed his soul instead of Attrebus'. The last thing Sul heard was Vile maniacal laughter.