Strongly disagree with a lot of what Matt says, having played MBC for the last four months in Standard. However I think that for the curve's sake you could remove a Wurmcoil, though, maybe for a Duress or a Mind Sludge (as a tutor-target from Liliana).

3 Inquisition, 1 Duress is fine - I played it for months, the vast majority of UB Control players are doing it now. A lot of those lists also play an extra Duress in the board, though, which I am also currently doing.

Skinrender is an awesome SB card but is really bad against control. He's a good tempo play, but when you can't slam him on turn 4 or 5, he doesn't do anything unless you're trying to put one of your guys on a Mimic Vat (which you're not even doing here). On that note, I'd play Skinrender over Hideous End in the board.

For consistency's sake, I'd lose the Specters for another Gatekeeper and another Sign in Blood. I would never, ever play this deck without 4 Sign in Blood - it's easily one of the best cards in the deck. Gatekeeper is amazing and especially good here because you're playing Percy, so auto 4-of. Specter is okay but not better than Nighthawk or Gatekeeper.

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I think you could lose a Zenith for a land. Hitting six mana every single game for this deck is really important, and I was happy with 25 lands and 4 Sign in Blood for a long time, under what was basically the same mana curve. Also, considering Zeniths shuffle back, you don't want to draw too many in the late game (because after you cast the first one, you probably won't need to cast more than one more for the rest of the game - your opponent can't come back from back-to-back board wipes that many times in a game.)

I think Memoricide / Sadistic Sacrament is better in the SB than Contaminated Ground. Without Tectonic Edge (which you can't afford to play, because of Corrupt) you won't draw enough mana denial to really screw the opponent. Memoricide just ruins everyone's day, and prevents the opponent from casting problematic spells in a more direct way than Contaminated Ground does.

Don't meant to shape your list into what I played, just hope to share some experience. There's a lot of room to play around here, so just test and build on what you like!

take out 2 Corrupts and 1 Wurmcoil Engine for 3 Skinrender. that will help your mana curve.

sitck to Duress, Inquisition of Kozilek will very rearlly take away needed critter from their hands UNLESS you have 4 in the deck. If you want to kill weenies then use Smother  or Grasp of Darkness. Grasp also has the added ablity to kill indestructibles and large fattys with the aid of -1/-1 counters from Skinrender or Zenith.

also Carnifex Demon + Skinrender /Black Sun's Zenith = double your pleasure, double your fun with doublemint gum. also infect -1/-1 can be passed too.... hint, hint.