I've been working on Grixis Eldrazi for several months now ever since BFZ came out, and with a ton of tweaking I think this deck has finally reached a point of strong viability. The previous iteration of the deck is still pretty decent as well, though it plays very differently and is premised on a different theory.
The purpose of the deck is to control the early game using Horribly Awry (a highly underrated and underutilized card), Spell Shrivel, and Thought-Knot Seer long enough to build up your mana base with Herald of Kozilek, Matter Reshaper, Shrine of the False Gods, Oblivion Sower, and Conduit of Ruin until you can cast your bigger cards, like Reality Smasher, Breaker of Armies, Ulamog, and Kozilek.
The first several turns are very tempo. Whenever possible, on your first turn, use a fetch land to grab Sunken Hollow (or Smoldering Marsh if you already have a Blue source in-hand) and have it enter the battlefield tapped. On the second turn, one of the strongest set-ups is to play a Mage-Ring Network as your land drop for turn 2 and have a Horribly Awry in-hand and sit on that board state as long as possible, building up charge counters for your later-game plays.
Through hundreds of playtests, I have determined that it is more often than not better NOT to play your creatures while you have counterspells in hand. It is far better to take a few points of early game damage and slow down your opponent's game, as you will be aiming for your win cons around Turn 8 or so.
To make this more achievable, cast Herald of Kozilek ASAP. Again, you should generally not do this before you are out of counterspells, though one of the best plays is to cast Herald and have one blue mana open with a Horribly Awry in and. Truly a cruel set-up. After Herald, try to get Matter Reshaper out and use him as a blocker/reckless attacker to get him in grave ASAP to cycle through your deck, fill grave for Murderous Cut, or of course, cheat out lands/Herald/another Matter Reshaper.
By Turn 4, regardless of your hand or board state, it is usually best to cast Thought-Knot Seer to get an idea of what their hand is looking like and how you should start playing your future turns. The big body on Seer means that whatever weenies they have managed to slip through won't be a threat for long.
By Turn 5, if you've been able to charge your Mage-Ring Network(s), you should be able to cast Oblivion Sower or Conduit of Ruin a turn early, and getting that kind of ramp at this point can make the game run a lot smoother for you. If you know you can cast a Titan by the next turn, it's worth stacking it with Conduit, especially if you already have a land to drop during the next turn. If not, Breaker is a solid back-up plan, clearing the board and slowing down the game to make your win cons easily achievable. Breaker of Armies often serves as a board wipe on opponents' established board states. Even if he dies in the process (which happens less than you might think), his magnet blocking makes all your other stuff essentially unblockable, making way for an easy win. Alternatively, if you weren't able to cast a Turn 4 Reality Smasher, Turn 5 is as good a time as any.
Finally, by Turn 7 or 8, you should be able to cast a titan, or perhaps even Ugin (who serves as another board wipe), but even if you can't your utility lands (Mage-Ring Network/Tomb of the Spirit Dragon) can help keep you in the game and give you a good place to dump your mana until a win-con presents itself.
Some of my favorite combos in the deck is Matter Reshaper dying and pulling out another copy of himself or Herald of Kozilek, but even just an extra land is nice ramp on its own. Herald of Kozilek into Conduit of Ruin, Ruin search Breaker or Ulamog, and then using the mana ramp to cast a titan is pretty great too.
Also, I'd recommend using Murderous Cut on big stuff you can't otherwise get rid of, such as a highly-threatening flier or big body (Linvala...cough cough...Omnath..cough cough). The mid range stuff like Icefall Regend is actually not that threatening, as Wastland Strangler can kill it just fine, and is easy to get on board.
I currently am trying to find a way to fit 2 of Jace, Vryn's Prodigy into the deck, but can't seem to find the space.
Other runner-ups include Ulamog's Nullifier, Bearer of Silence, Fathom Feeder, and Hedron Archive.
I keep wavering on whether or not to keep Kozilek in the deck or to replace him with another Ulamog. Ulamog serves as a more potent win condition, and not only removes two threats from the board, but puts the opponent on a very short 2-turn timer, and is indestructible to boot. Kozilek almost always makes it to the board, and his countering ability is more useful than I'd expected (what with all the 3- and 4-drop cards in the deck), but he simply doesn't put the pressure on an opponent in the same way. More than anything, I view him as a cheap and easy-to-cast draw spell, as the deck does tend to run out of cards by turn 8.
I'd really like any advice you have to offer. Specifically, I'm in great need of Sideboard suggestions, as Jeskai Prowess constantly gives this deck a tough time, seeing as Swiftspear tends to hit the board before Horribly Awry is ever even live.
