This is my creation, with a little help from the Gatherer comments section on synergy for Thraximundar.
I got the shell for a Grixis deck for Christmas one year, but the mana-curve was not good so I scrapped it. Now, I'm taking the cool parts of it (Specters, Grimblades, notably the pair of Thraximundar, Defiler of Souls...)
The main idea of this deck is controlling your opponent until you're able to activate the bombastic combos. Use cards like Kederekt Creeper and Grixis Grimblade to keep your opponent from attacking, and use Sedraxis Specter to keep your opponent's hand thin. You have cards like Terminate and Spawning Breath to pick off some potential threats early.
Spawning Breath ties in to the next theme, the creation of Eldrazi Spawn to ramp into things like Corpsehatch and Spawnsire of Ulamog, a good mana sink as well as a win condition (provided you own a few Eldrazi, of course!).
Thraximundar finds excellent synergy in the deck as well, as the Eldrazi Spawn are now- Sac: Add [1] to your mana pool, and give Thraximundar a +1/+1 counter. It That Betrays works well with Thraximundar's forced sacrifice, and it's always fun to steal your opponent's stuff, anyhow.
There are some utility cards like Slave of Bolas, and Suicidal Charge. The former is a fun way to kill your friend's creature, and Suicidal Charge is interesting in that it can leave your opponent completely defenseless, and it can force some unfavorable trades for them. It's a poor man's Master Warcraft.
A possible weakness of this deck is anything that flies, but hopefully you can manage to kill those before they're terribly threatening. Anything on the ground will die soon enough, after all. Happy hunting!
