Death, disease, decay, have been lurking since the Oblivion Crisis. An army of the dead has arisen, soldiers, mages, archers, assassins, the necromancer's army hasn't discriminated. Disease has spread like a wildfire, at first its barely noticable, you get a little tired quicker, until you start coughing up blood, then your muscles weaken, and die. It is a plague that is destroying many. Death is everywhere, Ocato tried to stop the mysterious "kidnappings" that have been occuring, but to no luck, it is assumed the dark brotherhood has been killing them, but the mages' guild believes it the necromancers. Ocato has sent out legionairres across the provinces to look for the hideouts of these kindappers, but to no luck, he is beginning to lose hope, and has taken measures to protect himself, by quarrantining the Imperial City.
Morrowind has been hit hardest by the necromancer army. The Temple (or what remained of it after the crisis) has been exterminated. House Redoran has been missing many numbers. House Hlaalu has lost only a few members. House Telvanni has lost none. The imperial legion is working hard to discover the culprits, but they are failing. Vvardenfell has become a wasteland, the vibrant green forest-like lands and the mushrooms have all died and decayed. The island is filled with undead, both humanoid and creatures.
Cyrodiil hasn't been hit hard, probably because you can't get in. You cannot leave or get in, due to the legion guarding the border. The imperial city is only available to Ocato, the Champion, and a few others. It is guarded by the legion's finest. If the necromancer army continues with its assaults on Tamriel, it will weaken it even further.
The rest of the provinces have barely been touched, a few undead have risen, but very few have come more than normal.
The leader of their army is an ancient lich, born thousands of years ago (about the same time as Divayth Fyr), and he claims to have heard the voice of Mannimarco, telling him to raise an army to help him end the miserable life of Arkay. Ocato has called all adventurers forth to the white-gold tower to learn the battleplans.
Edit: Fixed