The Greymarch is the crisis currently and presently affecting all Planes of Oblivion. Following his defeat by the Champion of Cyrodiil, Jyggalag was able to leave the cycle that had trapped him in the Shivering Isles, and was unleashed into Oblivion. In the proceeding two centuries, he began to spread his influence throughout the Planes once again. He begin this new Greymarch in 4E 41, once he had marshalled his strength and gathered his lieutenants: Lord Rahn, Agemon Tallking, and Arilemma.
As it was so similar to what would occur to the Shivering Isles once in an Era, this slow march for control of all of Oblivion was also called the Greymarch. How Jyggalag exerted his power and influence manifested in ways extremely similar to the eponymous event. If left unchecked it is destined to spread throughout the entirety of not just Oblivion, but later into Mundus and eventually Aetherius itself.
In 4E 210 Azura assembled a group of mortals in a bid to halt the Greymarch.