The Eldrazi are massive soulless monstrosities whose exact origins are unknown. Many millenia ago the Eldrazi occupied territory in the aether itself, blocking planeswalkers and letting them suffocate within the Blind Eternities, as even planeswalkers cannot survive very long in the aether. Three brave planeswalkers including Sorin Markov lured the Eldrazi onto the plane of Zendikar and trapped them in a complex prison that can only be unlocked if any three planeswalkers enter it.
Now on Zendikar, the three planeswalkers Jace Beleren, Chandra Nalaar, and Sarkhan Vol accidentally released the Eldrazi from their millenia-long slumber at the Eye of Ugin. After aeons of patiently waiting for three foolish planeswalkers to unlock their prison, the Eldrazi storm out from their resting place and lay waste to everything in their path with Eldrazi from the brood lineage of Ulamog crushing all that lives, Kozilek destroying the landscape, and Emrakul altering the very reality around them.