Planemeld

The Planemeld was a massive Daedric invasion of Tamriel in the 6th century of the Second Era, during the Interregnum. The Daedric Prince Molag Bal used his Dark Anchors, huge Daedric interplanar machines, in an attempt to drag Nirn out of Mundus and into his own realm of Coldharbour.

Varen's Rebellion
In 2E 573, 5 years before the Soulburst event, then-Emperor Leovic of the Longhouse Emperors legalized Daedra worship throughout the crumbling Empire of Cyrodiil. After hearing of this, Varen Aquilarios of Chorrol summoned the military forces of the Colovian Highlands to depose Leovic. Varen based his rebellion out of Bruma, from where he coordinated his troops as they liberated Cyrodiil's other cities to further weaken Leovic's control. The rebellion claimed victory when Varen's forces stormed the Imperial City, and Varen personally dispatched Leovic at the foot of the Ruby Throne before proclaiming himself emperor.

The Five Companions
After Varen claimed the throne, he wanted to legitimize his claim to the Ruby Throne and thus end the Interregnum, which could only be done by a Dragonborn. One of his Five Companions, the necromancer lord Mannimarco, convinced Varen he knew a ritual which would allow the emperor to become Dragonborn. To do this, however, they needed the Amulet of Kings.

Thus, the Five Companions were formed: Varen Aquilarios himself, Mannimarco, Lyris Titanborn, Sai Sahan and Abnur Tharn. They set out on a four-year quest to find the Amulet of Kings.

The Soulburst
The Five Companions able to retrieve the Amulet of Kings and begin the ritual in the year 2E 578. The Five gathered at the Temple of the One in the Imperial City, and they began to perform the ritual. At that moment, Mannimarco used his dark magic to immobilize the others and reveal his true intentions: he was an agent of Molag Bal, and the ritual in fact weakened the barriers between Nirn and Oblivion in an event known at the Soulburst.

This event allowed Molag Bal to initiate the Planemeld, a forceful merging of worlds. Once the ritual was completed, aftershocks were swept across every corner of Nirn. Mages died or went mad, Red Mountain trembled in Morrowind, earthquakes shook Skyrim and heavy storms devastated the coastlines of Elsweyr and Valenwood. The constellation of the Serpent grew so large that it seemed to threaten every star sign at once.

After the Soulburst, Varen mysteriously went missing. Lyris was captured by Mannimarco to be sacrificed, Abnur fled and Sai Sahan fled with the amulet, managing to hide it in Sancre Tor.

Afterwards, the Tharn family, who were the rulers of Cyrodiil through the new Empress Regent Clivia Tharn,[6] made a pact with Mannimarco, The King of Worms, who had agreed to supplement the Imperial's forces by resurrecting their soldiers, except for Clivia's father Abnur who deeply despised Mannimarco after the Soulburst.