(http://wiki.dominionstrategy.com/images/1/1f/Feodum.jpg) (http://wiki.dominionstrategy.com/index.php/Feodum) (http://wiki.dominionstrategy.com/images/c/c1/Herald.jpg) (http://wiki.dominionstrategy.com/index.php/Herald) (http://wiki.dominionstrategy.com/images/6/63/Noble_Brigand.jpg) (http://wiki.dominionstrategy.com/index.php/Noble Brigand) (http://wiki.dominionstrategy.com/images/c/cb/Spy.jpg) (http://wiki.dominionstrategy.com/index.php/Spy) (http://wiki.dominionstrategy.com/images/1/10/Duke.jpg) (http://wiki.dominionstrategy.com/index.php/Duke) (http://wiki.dominionstrategy.com/images/e/ed/Tactician.jpg) (http://wiki.dominionstrategy.com/index.php/Tactician) (http://wiki.dominionstrategy.com/images/e/ef/Venture.jpg) (http://wiki.dominionstrategy.com/index.php/Venture)
(http://wiki.dominionstrategy.com/images/3/34/Mission.jpg) (http://wiki.dominionstrategy.com/index.php/Mission) (http://wiki.dominionstrategy.com/images/d/df/Seaway.jpg) (http://wiki.dominionstrategy.com/index.php/Seaway) (http://wiki.dominionstrategy.com/images/f/fa/Black_Market.jpg) (http://wiki.dominionstrategy.com/index.php/Black Market) (http://wiki.dominionstrategy.com/images/e/ec/Gear.jpg) (http://wiki.dominionstrategy.com/index.php/Gear) (http://wiki.dominionstrategy.com/images/5/59/Duplicate.jpg) (http://wiki.dominionstrategy.com/index.php/Duplicate)
Mission, Seaway, Black Market, Gear, Duplicate, Feodum, Herald, Noble Brigand, Spy, Duke, Tactician, Venture
https://dominion-game-logs.s3.amazonaws.com/game_logs/20160630/log.0.1467304725506.txt
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So the obvious thing here is Tactician-Black Market. I of course completely miss this, thinking hey, Gear and Herald look pretty good, and we can sprinkle in some Black Markets! Then instead of buying Heralds, I buy... other stuff for some reason.
Meanwhile, my opponent has gone Tactician-Black Market; they asked in chat if I had missed it or decided against! Then they get Mountebank and Haunted Woods out of the Black Market deck, which was almost doom... but Herald is the only village and the only good Seaway buy. So they end up hitting Tactician with Herald several times.
Obviously I made more mistakes than my opponent, though I wonder how much of my deficit was the Mountebank.