Main Strategy
Phenax, God of Deception is a very well known commander to run for a mill deck. There are different strategies with mill, the first being a parallel mill. This is where you mill all of your opponents evenly, however I find it to not be as effective because it only mills them a few cards at a time. With this deck you'll be focusing your mill cards on a single opponent, but will be milling them for multiple cards.
Phenax Mill
There are two versions of mill in this deck, the first is milling opponents with Phenax's ability. His ability gives creatures you control a tap ability to mill opponents based on the creatures toughness. Since it is based off of the toughness instead of power of a creature, there are multiple defenders in this deck. One of the best defenders is Tree of Perdition. This card has the ability to trade life with an opponent. If you get the card out early enough you can set an opponents life to 13 and have it's toughness become 30+. Then you can mill your opponents for 30+ cards with just the one defender. Running multiple defenders makes it so you don't have to run as many other creatures to focus on attacking. So it is important to have different types of defenders to give extra protection. Wall of Frost keeps opponents from untapping the blocked creature on their next untap step. Wall of Air and Hover Barrier have flying to block against flying creatures. Fog Bank is really helpful for protecting against opponent's bulky creatures with the ability to cancel any combat damage dealt to it. I like to use it to stop creatures with deathtouch because I can still keep my creature. Because the mill is a tap ability, it is better to keep your creatures untapped during opponents turns so you have ways to defend. Then you can use the ability on the end step of the opponents before you. Paradox Engine can be lethal in this deck because you can mill your opponents then play a spell to untap all nonland permanents. If you have the mana and cards, you can continue to mill, then play a spell to untap creatures with Paradox Engine. A card that works very similarly to Paradox Engine is Intruder Alarm. This card doesn't let anyone untap creatures on their untap step. Instead everyone untaps their creatures whenever another creature enters the battlefield. The key to playing this card, is whenever an opponent plays a creature spell, you mill your opponents with the Phenax ability while the spell is still on the stack. When the spell resolves your creatures untap. It is a way to continue milling your opponents whenever they play a creature spell but you still have creatures that you can in turn defend with. However it only works on ETB effect so if you mill while it is on the stack and another person then counters the spell, you will not be able to untap your creatures. So that is a way opponents can target you by countering another spell. Eater of the Dead combined with Phenax, God of Deception let's you mill opponents then untap him to exile a creature from an opponents graveyard. This combo can help you continue milling through entire libraries and getting rid of a lot of creatures that could come after you.
Direct Mill
The second version of mill is based on creatures that already have the mill ability or based on sorcery, artifacts and Enchantments. Mindcrank is a great artifact that mills opponents whenever they lose life. This includes if they lose life from themselves or from other opponents. Mind Grind is a finisher for this deck. This mills all opponents until they mill up to X Lands. When it starts coming to end game, opponents will have already played multiple lands and you will have enough mana to have X reach double digits. It is more ideal to play this card when you can have X stand for 12+. There are enough lands and mana rocks in this deck to help get enough mana to play this card for enough to mill all opponents completely. Ulamog, the Ceaseless Hunger is a card that should be included in every mill deck. Whenever he attacks he mills the opponent 20 cards but those cards are exiled instead of being put into a graveyard. This can be extremely helpful if an opponent has Eldrazis in their library as well.
Mana
This is a Dimir deck (Black & Blue). When running a Dimir deck it is a little more challenging to get the mana ramp that you need with mana rocks, however there are still a few options with Dimir Signet, Sol Ring, and Gilded Lotus. This deck runs more blue than black so there can be instances where your black mana is running low. Chromatic Lantern helps so your lands tap for any color, and Urborg, Tomb of Yawgmoth lets your lands tap for swamp in addition to their current type. Paradox Engine also helps build your mana base. When you have it on the battlefield, always play spells using your mana rocks because you can then untap the mana rocks, essentially playing the spells for free. There are plenty of lands included in the deck as well since there are less mana rocks for these colors. Some of the lands give extra advantage as well. Reliquary Tower lets you have no max hand size, and Academy Ruins let you get artifacts from your graveyard. Homeward Path can help a lot because opponents will try to steal your permanents. You can use its ability to keep your cards.
Card Draw & Tutor
Since there are so many defenders in this deck, you go through cards quickly because of the cheap mana cost. Rhystic Study can get you the extra card draw based on opponents playing spells. Ponder will help you draw for cards if you missed a land drop or don't have playable cards in hand. Mystical Tutor and Diabolic Tutor can help you find cards for combos or game finishers.
Recursion
With so many cards being milled, you have to have cards to play to take cards from graveyards. You can use Sheoldred, Whispering One to get a single create and have opponents sacrifice more creatures. Rise of the Dark Realms is another finish because it let's you get all creatures from all graveyards.
Punishment
Most people will include in their decks a way to play from their graveyard or shuffle it back into their library. Leyline of the Void exiles cards from graveyards, making it so opponents can't play from their graveyard, but won't let you play from them either. Duskmantle Guildmage will damage opponents from your mill effects.
Removal
Running Blue and Black, you can also run plenty of Removal and Counter spells. There are board wipes like Cyclonic Rift and Damnation. There are destroy target creature cards, Hero's Downfall, and counter spells like Disallow.
Protection
It is always good to run extra protection in a deck. Propaganda can help dissuade opponents from hitting you, or keep them from hitting you with as many creatures. Lightning Greaves can protect your commander or other heavy hitting creatures. Expropriate can be a game finisher because of the extra turns. Extra turns just give you more time to mill your opponents. Crypt Incursion can get you massive life gain by exiling creatures from opponents decks. Mill decks can be extremely powerful but easily countered as well if an opponent throws in a few Eldrazis. Laboratory Maniac is a second route for this deck to go by milling yourself to win the game. This should not be a go to strategy because Laboratory Maniac is a creature and can easily be countered or removed. However if it becomes difficult to mill your opponents or your opponents find ways to mill you, this is a second option to winning the game. Just make sure you have the cards and mana to protect Laboratory Maniac.
Conclusion
This is my first mill deck, so I am sure there are many different cards I can include or replace. Please give me your thoughts, opinions, and suggestions! I am always looking for different ideas and inputs. Overall I think it is a fairly solid deck and a fairly low cost. It's a lot of fun to play and a deck I would suggest building yourself! Thanks for looking at the deck and giving your advice!
