While still 100% rumor, a user on MTGSalvation has come forward saying that he came into possession of a foil sheet of Rize of the Eldrazi cards. Until a clear scan is seen, the following is to be taken with a very large grain of salt.

Abilities

[easybox]Annihilate X: When this creature attacks, the defending player sacrifices X permanents.[/easybox]

This seems extremely powerful, because it could quickly prove decimating to an opponent. Consider a mana ramp deck against a control deck, if the ramp has six mana available on turn three and attacks, the opponent could quickly find themselves without any lands. A one-sided armageddon. Admittedly this ability would most likely be on the gigantic Eldrazi spells we all expect to see in the set, so they would be prohibitively expensive to cast.

Update: I may have misunderstood this, it may be that X is a number that changes depending on the card and not a mana cost to be paid. That would make it MUCH MUCH more reasonable.

[easybox]Level up X (pay X, put a level counter on this creature. Level up only as a sorcery)[/easybox]

This echoes the themes of Zendikar's adventure and almost RPG style. Sort of echoing the quite popular Figure of Destiny. The concept being that with each level a spell does something different or more powerful. Imagine a leveling Prodigal Sorcerer, it would deal 1, then 2, then 3, etc...

[easybox]Eldrazi Temple

Tap: 1 colorless to your pool

Tap: 2 colorless to your pool. Spend this only on colorless Eldrazi spells or permanents.[/easybox]

This seems logical, a land which is basic but proves a bit more useful in powering out the titans of Zendikar. Nothing shocking, here.

Overall these all seem powerful and reasonable. If they prove to be true it also shows the earliest spoiling for cards ever.

Verdict: Plausible but needs proof.

Update 2:30pm EST: The original thread has been removed from MTGSalvation, because he deleted the original post was deleted. Whether this lends credence to the spoiler or not we cannot be sure.