The first thing I thought of when looking at this image at the Magic Art Show in Las Vegas
Mari, the Killing Quill is easy to cast, and with deathtouch, your opponents will be less likely to block your Assassins out of fear of losing their own important creatures
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Pair it with mass removal like Wrath of God or mana denial cards to make sure opponents never recover while you comfortably cast your bigger late-game spells
The Mono-Blue build can also exile Sleep-Cursed Faerie, but here the plan is to combine its untap ability with Omen Hawker
it also adds to your devotion to blue, which can be relevant for Master of Waves or Nykthos, Shrine to Nyx