The battle was fought between the Imperial Legion under Emperor Titus Mede I and Lord Vuhon's (aka Lord Umbriel) undead army, which was composed of re-animated Argonians, Dunmer, and Imperial legionnaires.
Arrival in MundusEdit
After bringing the floating city of Umbriel into Mundus from Clavicus Vile's realm in Oblivion, Vuhon destroyed the city of Lilmoth in Black Marsh to gather souls to power his Ingenium in order to keep Umbriel afloat, and to gather bodies for his undead army. He then directed the city northward through Black Marsh, destroying the cities of Stormhold and Gideon along the way before crossing into Morrowind, Vuhon's homeland. He hoped to find the Umbra sword in the ruins of Vivec City, as it was the only thing that could destroy Umbriel and himself. After capturing Sul and Prince Attrebus Mede and finding that the sword was no longer in Vivec, he turned the city southwest towards the Imperial City, sending his undead horde to besiege it before Umbriel's arrival.
Vuhon needed to bring the city to the White-Gold Tower in order to break Clavicus Vile's hold on the city, as it was technically still in a "bubble" of Oblivion, and the Tower, as one of the axes of the creation of Mundus, could emancipate Umbriel from Oblivion. He sent an undead minion to the Emperor with an ultimatum: he would permit everyone to leave the Imperial City unharmed, so long as they gave him the city. Emperor Mede refused and prepared the Imperial Legions for battle. In fact, Mede had been aware of Umbriel before it even attacked Lilmoth. Despite his son Attrebus' demands for action, Mede had refused since Black Marsh and Morrowind were no longer part of the Empire. However, he sent mages escorted by legionnaires to survey the city just in case it turned towards Cyrodiil. When it did, the scouts fell back as Umbriel destroyed Cheydinhal.
The siege Edit
Skirmish northeast of the cityEdit
Commanding the Legion's skirmish was General Takar, a Redguard from Hammerfell. He led his forces, five thousand soldiers made up almost entirely of cavalry and mages, northeast of the Imperial City to meet Umbriel before it reached the city. With Takar were eight wagons containing unknown engines. As the cavalry charged the undead lines, the mages took to the air with the wagons, flying straight at Umbriel. Within its bubble, Vuhon's flying "Taskers" appeared in huge swarms and slaughtered the mages before they could get close. After that General Takar sounded the retreat, and Umbriel proceeded undettered towards White-Gold Tower. The Synod and College of Whispers made two more attempts to attack Umbriel by air, without success.
Battle on the shores of Lake RumareEdit
Two days later, Umbriel arrived at the Imperial City. Several Legion detachments under Commander Prossos were deployed along the shores of Lake Rumare to attempt to hold the undead. While the disciplined legionnairies were able to kill several times their own number, the enemy kept on coming in endless waves. A batallion of spearmen was able to repel an attack on the main gates, but the undead turned their assault to the northeastern wall near the Imperial Prison where Umbriel was floating over the walls. There, the Twelfth Legion was pushed back to the walls of the city as countless thousands of undead surged against their shield wall.
Fall of Lord UmbrielEdit
Inside Umbriel itself, Sul, Attrebus and Mere-Glim discovered that Vuhon had stored his soul within the Ingenium itself along with the Umbra entity, and only by stabbing it with the Umbra sword would he be killed. Mere-Glim attempted to use the sentient Hist-like trees in the Fringe Gyre to move Umbriel out of Mundus, but Vuhon sensed his attempt and used his magic to prevent it. The trio managed to reach the Ingenium in the heart of the city, and while Mere-Glim and Sul held off Vuhon, Attrebus drove the Umbra sword into the Ingenium. Vuhon's soul and the Umbra entity were drawn into the blade. Attrebus was then possessed by Clavicus Vile, who planned on claiming his soul and taking Umbriel back into his realm of Oblivion. However, Sul attacked Vile, who stabbed him with the sword, causing Attrebus to let go and Sul to fall onto the Ingenium. Vile released Attrebus and claimed Sul's soul instead. With Vuhon dead, Mere-Glim was able to use the Hist-like trees to take Umbriel out of Mundus into the realm the trees originated, not Vile's realm.
As soon as Umbriel left Mundus, the undead army besiegeing the Imperial City simply fell where they stood.
The Imperial Legion was victorious, which resulted in the end of the Umbriel Crisis. While Attrebus and his companions were responsible for the end of the Crisis and the battle, he decided to give credit to the Synod and the College of Whispers, as the two organizations had worked together for the first time during the Crisis, and he wished for them to be on good terms for the good of the Empire.