Landfall is the name given to the future event in which Nirn is destroyed in the late Fifth Era during the Numidium's return to Tamriel.[OOG 1] It is stated that the only way to fully escape the devastation of Landfall is to know love.[OOG 2]
The origins of the event can be traced back to the ending days of Tiber Septim's conquests in which the Numidium was used. During the final battle of the Tiber's war against the Second Aldmeri Dominion, the siege of Alinor, the Numidium was caught in a time warp. While the siege of Alinor only lasted around an hour in perceivable time, the warp caused the Numidium to remain in battle with the forces of the Dominion from some point in the Merethic Era all the way to the late Fifth Era.[OOG 3] This distortion in time allowed the Numidium to essentially escape its own demise at the hands of the Underking.
After the Numidium's eventual return to Tamriel in the Fifth Era, it resumed its war against the elves, taking on the forces of the Third Aldmeri Dominion. During this time, a strange insect-like prophet who foretold the return of Numidium traveled to Morrowind in order to save its inhabitants from the coming apocalypse, but would be crushed by the Numidium before he could achieve his goals.[OOG 4][OOG 5]
While the Thalmor tried to fight off the Numidium their resistance was proven to be futile when the Numidium used its "ancestroscythe" abilities to effectively erase the Altmer race from existence. As the Numidium was about to annihilate a large group of Khajiit and Dunmer refugees attempting to escape the destruction in a ship known as the Wonderweir, the Nerevarine appeared, piloting the Akulakhan and began to battle the Numidium. With the Numidium distracted, the Wonderweir was able to leave Nirn, reaching the moon Masser where the passengers would take refuge and eventually create colonies under its surface. However, the Nerevarine was unable to stop the Numidium which would continue its destruction of Nirn and, after some time, would move on to Masser where it would be verbally defeated by a Dunmer known as Jubal Lun-Sul who would later go on to end the Landfall by creating the first of the "New Men" with Vivec.[OOG 5][OOG 6][OOG 1]