Well, there have been cards spoiled from Zendikar in the new Planechase pre-constructed decks. The following image shows 4 commons from the 4 different pre-constructed decks available in the Planechase products.
These are confirmed by Wizards to be Zendikar "preview" cards and not from another future set.
Whiplash Trap confirms a new set mechanic called Trap which is on an instant in this image (but could be on other spells/permanents - no news on this yet) while
Kor Sanctifiers confirms the return of kicker as a mechanic. An interesting thing to note is the use of the verb form of kicker in the form of, "if it was kicked". Sounds a bit funny and I'm sure everyone will get a few laughs from kicking things while they play this block. I can predict some horrific puns and bad jokes coming out of this. Also interesting to note is the return of the Kor. Current speculation is that Zendikar must be the Kor's home plane.
I love the artwork of the whiplash trap throwing a humanoid creature flying into the air. Great flavour-art combo.
Hideous End looks like it is affecting an Elf and the lose life effect supports the vampire tribe rumour. The Kors look like Kors so nothing major in that artwork but the
Beast Hunt looks mighty interesting since the beasts/behomoths look very reptilian in nature. We know they are not dinosaurs but they're looking mighty close if you ask me.
Looking at the flavour text:
Hideous End“A little dark magic won’t stop me. The worse the curse, the better the prize.” ―Radavi, Joruga relic hunter, last words
We can assume that Joruga is either a tribe/type of elf in Zendikar or it is a location name.
Kor Sanctifiers“Why keep such trinkets? They only add weight to your travels.”
Beast HuntNothing exciting to be gleamed from that text.
“Surely we could tame something besides hurdas and pillarfield oxen!” ―Sheyda, Ondu gamekeeper
As before, Ondu may be a tribe name or location name. And here's thinking that we have hurdas as new flavour-adding creatures similar to the mounts ridden by Lorwyn's races - the Kithkin springjacks and the Elf cervins (was it that?).
