ES Online: Storyline

The Elder Scrolls Online takes place in the Second, or Common, Era of Tamriel. The events of the second era take places around one-thousand years before the events of The Elder Scrolls V: Skyrim in which the dragons have been reawaken. Instead of the continued appearance of dragons and revolving around their storyline, the Elder Scrolls Online focuses on the Deadra and a massive worldwide race to Cyrodiil between the nine races and their alliances. It seems that the humans that rule the Imperial City (the human imperials (a non-playable race)) have begun to rely on their ties to the Daedra world to strengthen Cyrodiil.
Official Storyline Description:

It is a time of strife and unrest. Armies of revenants and dark spirits manifest in every corner of Tamriel. Winters grow colder and crops fail. Mystics are plagued by nightmares and portents of doom.

Four years ago, in 2E 578, an arcane explosion of energy in the Imperial City set off mystical aftershocks that swept across Nirn. Mages died or went mad. Supernatural abominations from the plane of Oblivion, the Daedra, appeared in greater numbers than ever before. The constellation of the Serpent grew so large that it dominated the night sky.

So began the grand scheme of Molag Bal, Daedric Prince of domination and enslavement. His Dark Anchors, vortexes of evil magic, weaken the barrier between worlds, threatening to merge Nirn and Oblivion into a single, nightmarish hellscape.

In the midst of this chaos, three alliances vie for control of the Imperial City and the White-Gold Tower. High Rock, Sentinel, and Orsinium stand as one, united under the rule of the High King in Wayrest,  Valenwood and Elsweyr have forged an alliance with Summerset, and Black Marsh, Morrowind, and Skyrim have joined forces and formed their own uneasy pact.

The Daggerfall Covenant. The Aldmeri Dominion. The Ebonheart Pact.

Three armies will take up arms against the Empire, and against each other, to wrest control of the Imperial City and White-Gold Tower from the dark forces of Oblivion itself.

Where do your loyalties lie?