Dimir Terror is a tempo-control deck built around cheap interaction and large threats like Tolarian Terror and Gurmag Angler that are hard to answer profitably. Duress strips their counterspells and Snuff Outs before you develop your board, protecting your key creatures like Kor Skyfisher and Glint Hawk. The additional Suffocating Fumes comes in to handle Sneaky Snacker tokens and to blank Mental Note/Thought Scour value if they go wide. Tithing Blade is too slow and mana-intensive against a deck that can counter it or ignore the life drain, and Omen of the Dead and the main-deck Mukotai Ambusher are low-impact in a matchup defined by tempo. Focus on flooding the board with cheap artifacts and bodies to outpace their one-for-one removal while using Duress to clear the way for your threats.