It has been a very long time since anyone in the plane of Zendikar saw the eldrazi titan Emrakul.
Visions of a devastated land trouble Gideon Jura, who has decided to track down the last of the three monstrous giants. With Jace off to Innistrad and Nissa healing the plane along with Chandra (However she can), Gideon follows the essence which Emrakul left in her last hedron prison.

This leads him to a peaceful, new plane filled with strangely sentient creatures. However, Gideon can't allow himself to be distracted by this, since as every second goes by, the eldrazi titan becomes stronger and spawns a huge brood of all-consuming aberrations.

If you play MTG, but don't follow the story, here's the summarized story of the Zendikar block:
First
Second

You can read this story without knowing the MTG lore. But it's highly recommendable to do so in order to understand many keywords.