The official visual spoiler for Rise of the Eldrazi is out
Wizards Visual Spoiler
Wizard articles you might have to read:
Creatures On Another Level - talks about the new "level up" creatures.
On the Rise - Mark Rosewater talks about the set design.
Below is taken from Wizard's Rise of the Eldrazi product page:
But unknown to the plane's denizens, there's a sinister reason for Zendikar's danger: for millennia, the plane has served as a prison for the Eldrazi, astral monstrosities native to the Blind Eternities. Now, the perils facing adventurers on the plane of Zendikar have taken an even deadlier turn. The Eldrazi have been released.
Hailing from the Blind Eternities, the space between planes, the Eldrazi have transcended the colors of mana as we know them. As a result, the Eldrazi progenitors themselves and those closest to them are colorless. But don't let the mana cost fool you—these Eldrazi are not artifacts.
Each of the colossal Eldrazi spawns its own "brood lineage." The three brood lineages are composed of Eldrazi Drones and other subordinate beings, each reflecting the image of its progenitor. The smaller Eldrazi Drones that are aligned with colored mana and many of these drones, as well as some spells, produce Eldrazi Spawn creature tokens. These 0/1 colorless creatures can be sacrificed to add one colorless mana to your mana pool—perfect for casting those enormous Eldrazi.
The Rise of the Eldrazi set also marks the debut of colorless instants and sorceries. Like their Eldrazi creators, these versatile spells can be put into almost any deck.