So: Eldrazi. Killing Modern, Legacy, and to some degree, even Vintage. So: We fight Fire with Fire.
12-Post is a deck that focuses on using lands called "Locuses" and Vesuva to generate tons of mana, sometimes 16 mana, just from lands, by turn 4. All of this mana is colorless, however, so how about we go about using it to cast some of the best creatures in the game: Eldrazi.
The deck consists of 4x Cloudpost, making our locuses 1-4. These are the basis of the deck, as well as 4x Vesuva and 4x Glimmerpost, making our locuses 5-12. Vesuva is very important, copying our Cloudposts and Glimmerposts to stabilize.
This is Eldrazi. So, to make us have consistent threats, how about we go with 1-2 of Eye of Ugin. This will help us keep our threats coming, and ramp at the same time.
We can play up to 16-post, but let's keep it at 14. So, let's add 2 Thespian's Stage. This can copy an opponent's Karakas or Wasteland, neutralizing opponent's threats or disrupting their mana base.
In order to always have our Locuses, we need 3-4 Expedition Maps mainboard. These are locuses 13-16.
So: We have 7-8 land slots left. Depending on the version that you are playing, you can replace these with basic lands, but I would suggest 6 Plains, 2 Karakas if you're going mono-white, 5 Wastes and 2 Karakas for colorless, and 3 Plains, 3 Forest, 2 Karakas for White/Blue.
The Big Dudes:
Our threats are, mentioned above, Eldrazi:
4 Thought-Knot Seer.
This guy is amazing. It can eat their combo piece, screw them for gas, and is good in the red zone. An auto-include.
3-4 Oblivion Sower.
What isn't to love about this guy? He steals their lands, eats their deck, swings with a huge butt, blocks well...
2-3 Wurmcoil Engine.
Amazingly deadly. Can stabilize, swing into a wall pretty safely, and outclasses even Gurmag Angler.
You can toss in 4 Eldrazi Displacer for white. Me? I would just board into these.
2-3 Emrakul, the Aeons Torn.
Our big finisher. Does much more need to be said?
4 Warping Wail.
I cannot say enough about this card. It kills Deathrite Shaman, counters Scapeshift, Tendrils... and makes tokens to boot!
1-2 All is Dust.
Wipes their board, and not yours! Swing in, usually for the win. This card just seems like the perfect stabilizer, and win con, for this deck.
The rest of the deck is pretty manageable, with cards from Top to Ugin. Just remember to play card advantage AND threats. Having just one is a disaster!
4x Crop Rotation.
This card is a cancel to Wasteland, as well as a ramp spell. Not much more to say.
There are many sideboard plans, from backup Show & Tell to just plain hate. I prefer to board in Engineered Explosives, Chalice of the Void, and some graveyard hate.