I do not see withered wretch and extirpate and planar void working together well. Yes, they all attack the graveyard but if you use extirpate just use a an extra dark confidant main and three null rods main. I would add a stifle and extracts for the chalices and two echoing truth for the skeletal scrying and chain of vapor.
As I've said before, Extripate is for attacking the engine. It works fine under planarvoid because planar void is triggered (so the card is in the graveyard for a second) unlike its leyline counterpart (which is a replacement).
Roll of Extripate: Hit 4-ofs, Brainstorm, Thirst, Drain, Force of Will... Those are the best targets. IF you think you can get away with it, hitting fetchlands or Duel lands and cut off access to offcolor mana.
Roll of Planar Void: You can't extripate everything. Planar Void also takes the mana stress off Wretch.
Roll of Wretch: Wretch is the final leg of the tripod. Without Planar Void, he pin-point's restricted cards in the yard that you wouldn't want to waste an extriapte on. And with Planar Void in play, he is a janitor who cleans out all those cards that made it. Also with planar void in play, he can trump your opponent's ability to have instant effect responses to the void trigger on the stack (such as necromancy or welder). Also Wretch is a win condition. Lastly, Wretch means your opponent cannot just bounce the planar void and then do insane things.
Roll of Extract: Extract and Wretch have similar rolls. They clear out dangerous On-ofs and restricted cards. Wretch hits the yard, and Extract hunts within the deck.
Why care so much about the yard? Every deck uses the yard in some degree. Spanning from Graveyard.dec like Ichorid, Dragon, or reanimator - to "not alot, but very important" like Slaver, Gifts, Crucible/strip or Rit combo - to "minor importance" such as powering a 4/4 beater for 2 mana or feeding a lavamancer.
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Another thing I would highly consider is running Shadow of Doubt over Stifle. Both serve the roll of attacking fetchlands, but additionally Shadow of doubt is amazing agianst Drain Decks and rit combo. The drawback ofcourse is keeping up 2 mana as opposed to 1. So in the longterm it may be worse than stifle, but it is certainly worth testing.
--------- About the Board--
- 5 "auto includes" -
1 Rod
1 Extripate
3 Energy Flux
- 10 fish cards-
3 Circu Dimir Lobotomist
3 Shadowmage Infiltrator / Dimir cutpurse
2 Darkblast
2 Psychatog / Nantuko Shade
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I played U/B fish when Rav had just been released. I played a Circu on the main and I found him to be INSANE against fish. Firstly he is a 2/3 so he can make quick work of any 2/1 or 2/2 that they attack with. Secondly he punishes consistancy, wich is what fish strives to achieve.
I also really like Tog in U/B fish vrs fish. His manaless pumping makes him great in combat. Aslo he thins out your yard to help starve out Grunts quicker. Shade in this slot is do-able, but with a 2/1 body you may find he over-kills creatures too much just to stay alive, and he ties up resources in the process.
Lastly, In the board, I think Shadowmages are the right choice over cutpurse. Cutpurse is a 2/2 and is very blockable. Shadowmage is a 1/3 and has fear. The discard against fish is often not important because fish doesn't really try to keep handsize anyway. For the most part, the only creatures in fish that can block this guy are confidant (big chump) and Curpurse.