This is my second attempt to make a blue deck, even more a Mill deck. Anyways, here are the key cards and reasons to use them:
[Fraying Sanity]: This is the main engine driving the deck. Every turn, the opponent will mill the same number of cards that were put in his grave that turn, this includes cards that came from the field, sorceries and instants he used. It means that as long as you have this stuff attached to your opponent, everything he does that sends something to the grave will screw him even more. But why not to help?
[Compelling Arguments]: Simple yet effective. Mill your opponent for 5 or discard it for one card. Later in the game you can recover this with another spells and creatures. At worst, you mill your opponent for 5. At medium, you mill them for 10 with the curse. At best, you play four of these and mill your opponent for 40. At the very best, with 3 curses, you mill them out.
[Fleet Swallower]: Self explanatory. If you get this in your field and have a curse in play, one attack will end your opponent. Either they keep this thing from attacking with something they probably aren't running, or they counter it, or you win. Run at 2 because expensive and this deck is blue and has no ramp.
[Seer of the Last Tomorrow]: Sneks, the least represented creature type in Amonketh and HOU (safe for birds, they were completely forgotten), this ones makes her way in our deck. 1 blue, tap, discard, mill 3. Considering the fact you will stop wanting lands at about 6 to 8 lands, you will pretty much discard those like the cardboard they are printed into. Mill for days.
[Minister of Inquiries]: Blue in Kaladesh and AER was one of the things I hated the most and this is part of the reason why. Gain two energy, pay one energy, tap, mill three. This deck has not many means of generating energy so unless you have a [Winds of Rebuke] in hand to either save this guy from a targeting or after blocking, you won't have much energy running, so this is why we have only three of them.
[Scribe of the Mindful]: One of the ways to keep your flow of mill trough Compelling Arguments + Fraying Sanity running. You will mostly use her to block then sac her to get the Compelling Arguments back and mill.
To end, we add some drawing engines and other things that blue most commonly has, that means counter spells. We are not running [Navigator's Ruin] because it requires your to attack in order to trigger it and you most often then not will not be attacking. This deck is mostly defensive. Might as well include [Gleaming Barrier] to add both mana and defense.
