Make Divinevert pull his hair out, then tell him "before you pull your hair out, I have a response".

Introduction:

Of the several different ways to win a Magic game, Mill uses the rare "make opponent draw from empty library" method. It is a relatively uncommon strategy that usually uses UB colours.

Why play Mill?

Mill is a not a good deck. (Yes, I said that.) It's short a few cards to actually be good. But it's got one major advantage over every other deck in Historic: it is very hard to defend against. Mill doesn't care about life totals or board presence or even traditional card advantage. Cards such as Soul Warden and Invoke Despair are significantly worse than normal against you, even laughably bad. It's fairly common that opponent does what looks like very weak plays in spite of a full grip, because Mill blanks so many cards.

Mill's bad matchups are fast aggro - the faster it is the worse it gets. Combo strategies tend to be pretty bad too, since you only have so many interactive spells that you might not draw. Some graveyard strategies are also bad matchups, because you fill their graveyard for them. Finally, there are some cards you just cannot beat if they resolve, such as Jace, Wielder of Mysteries and Clear the Mind in their deck (who the hell plays this card anyway? But if they do, you are in trouble).

Mill's good matchups are any deck with a slow clock. It's not just control - midrange decks like Sacrifice are also usually too slow to beat you.

Example deck:

1 Lurrus of the Dream-Den

4 Ruin Crab
4 Fatal Push
1 Cling to Dust

4 Drown in the Loch
4 Mesmeric Orb
4 Glimpse the Unthinkable
3 Maddening Cacophony
3 Founding the Third Path

4 Tasha's Hideous Laughter
3 Witch's Vengeance

2 Into the Story

3 Island
3 Swamp
1 Castle Locthwain
2 Fabled Passage
4 Watery Grave
4 Darkslick Shores
2 Ipnu Rivulet
4 Clearwater Pathway
1 Otawara, Soaring City

These cards are core and I believe you should always have 4 copies.

Ruin Crab: It's tempting to run 0 just to blank the opponent's creature removal, but you don't have enough card quality to eliminate Ruin Crab entirely from your list. The good news is that even if opponent has creature removal, Ruin Crab costs 1 mana, so you never lose tempo in the exchange. It's also able to block certain smaller creatures.

Mesmeric Orb, Glimpse the Unthinkable and Tasha's Hideous Laughter: Your attack cards. They mill enough that there's no reason not to have the full four. Mesmeric Orb has the incidental benefit of milling you as well, which can enable Founding the Third Path or Cling to Dust or Lurrus.

Drown in the Loch: Removal that doubles as a counterspell. Sometimes your only defense against graveyard decks.

Fatal Push: Staple removal spell. Some matchups it does nothing, but other times it's the card you most want to see. You can occasionally enable the Revolt with Founding the Third Path or Fabled Passage or even Ipnu Rivulet. Try to save your Fatal Pushes to answer the really key pieces your opponent might have. I've lost tons of games due to killing a non-essential piece; in fact the more games I play the more convinced I am that "bolting the bird" is not correct with Mill. You really need to kill e.g. Elvish Archdruid, Woe Strider (in conjunction with synergies like Samwise Gamgee + Cauldron Familiar), etc. The challenge is to identify what these key pieces are and hold mana up on appropriate turns.

Other considerations are:

Into the Story: 4 mana draw 4 is strong, but it's clunky in multiples.

Founding the Third Path: This card is weird. The turn you cast it, it lets you cast a 1- or 2-mana spell, then the third chapter lets you cast that spell or any other spell in your graveyard again. Do not play this unless you have an appropriate target for the third chapter. This could be either a spell that's already in your graveyard, a spell that you are casting with the first chapter, or something you are planning on casting during the second chapter. Note you have to pay mana for the third chapter, and you won't have the chance to make a land drop first.

Lurrus of the Dream Den: You often won't have the mana or time to use him, but none of your permanents (except one, below) costs more than 2, so this is effectively free. You do have some permanents you could recur with Lurrus, but don't count on blocking with him because opponent is almost certain to have spare removal spells.

Jace, the Perfected Mind: Mills for a reasonable amount, draws cards, potentially gains life. But Jace has several problems. First, he conflicts with Lurrus. Second, you almost surely won't have enough board presence to activate him twice. Third, it's actually not trivial to trigger the "draw 3" mode on his -2, and probably impossible on turn 3. This is partly because Tasha's Hideous Laughter exiles instead of puts in the graveyard. I am of opinion that Jace is not better than Into the Story, or at least not by enough to give up Lurrus as a companion.

Maddening Cacophony: Strictly inferior to Glimpse the Unthinkable. The point is that by the time you get to six mana (if you get to six mana), it's quite probable that opponent's deck will be low enough that the unkicked version actually mills more cards. It's a sign of your poor card quality that you probably have to run some copies of these anyway.

Cut Down and Sheoldred's Edict: More single-target removal if you need it. It's usually worse than Fatal Push. You could also play Sheoldred's Edict to kill single big creatures and/or planeswalkers.

Ritual of Soot, Witch's Vengeance and Languish: Your sweepers. Sometimes they are the only thing standing between you and aggressive decks. Witch's Vengeance deserves a special mention because it is 3 mana. Quite a few decks in Historic BO1 are highly vulnerable to Witch's Vengeance, including the Dreadhorde Arcanist deck, Merfolk, and Elves. Especially on the draw, you simply don't have time to get to 4 mana against these decks (if you get to 4 mana, since you usually don't play enough lands to consistently reach 4 mana). On the other hand, it doesn't usually kill enough against non-tribal decks. Ultimately it's your call.

Soaring Thought Thief & Thieves' Guild Enforcer: The rogues package has some advantages and disadvantages. These cards can potentially attack planeswalkers off the battlefield, and moreover Thieves' Guild Enforcer can also act as copies 5-8 of Fatal Push. Against that, they turn on the opponent's creature removal, and with at most 8 rogues in your deck you don't have enough rogues to reliably trigger them. They can potentially attack for considerable damage, but considerable is not decisive, and you don't win by damage. In terms of the real "damage" that you care about (i.e. cards milled) they usually mill less than your other attack cards. I believe they are inferior as a result.

Cling to Dust and Soul-Guide Lantern: Maindeckable graveyard hate. You have some amount of self-mill (Mesmeric Orb especially; some of your other mill cards can also self-target) that can help fuel/find the former, while the latter can be recurred with Lurrus. Warning: it's a lot of mana to put Lurrus into hand, cast it, and then cast Lantern.

Bruvac the Grandiloquent: As an attack card, this card has several problems. First, it doesn't do anything if it dies immediately (unless you already have Mesmeric Orbs & Ruin Crabs on the board). Second, the turn after you play Bruvac, he effectively becomes a copy of your next attack spell, which is not especially good. Third, Tasha's Hideous Laughter does not mill. Finally, running Bruvac means you can't run Lurrus. This card never got off the drawing board for me as a result.

Vantress Gargoyle: It's ridiculous to think you can activate Vantress Gargoyle 8 times so he can be as good as Maddening Cacophony (which is also strictly inferior to Glimpse the Unthinkable), so the only real reason to run him is to be an early-game blocker that can attack planeswalkers. But if you're going to do that, just run more spot removal and/or sweepers. Note there are few planeswalkers you actually care about. If they tick up enough then you lose, but if you're casting attack cards then it doesn't matter if they're ticking up.

Land choices:

Most lists run 23 lands, but I personally play 24, because the fourth mana is very nice to have. It's necessary to cast Into the Story, and you also have Lurrus as a mana sink.
There're a few utility lands to consider:

Fabled Passage: Main purpose is the synergy with Ruin Crab, but it's a CIPT land that's very painful during the first few turns. I am not convinced you want the full four copies.

Field of Ruin: Way too slow, it's not like you care about creaturelands in general and not producing U or B mana is a major drawback as well (given your UU spell [Tasha's] and potentially BB spells [the sweepers]). Run 0.

Ipnu Rivulet: 2 copies is common. Taking damage to tap is not good, but it can push through the last few points of "damage". Note this can trigger Revolt for Fatal Push.

Otawara: It's literally free. Takenuma is also free, but you're never activating it anyway so it's not a big deal.

Castle Locthwain: It's almost but not quite free, and the upside can be relevant. Better than Castle Vantress in general.

Gameplay:

There's not much to say about Mill gameplay. You are not very interactive, you just want to cast attack spell after attack spell until the opponent dies. You interact if you have to, and only hold up mana when you sense a game-winning spell coming from the opponent (like Unburial Rites against a reanimator deck).

Your nut draw looks something like T1 Ruin Crab, T2 Founding into Glimpse, T3 Tasha's, T4 Glimpse (via Founding) + another Glimpse. Turn 4 lethal is possible with this deck.

Some tips:

- If you're new to the deck, read Founding the Third Path carefully, because it's the most complicated card in the deck.
- Be sure to count your damage vs. your opponent's library. If opponent is on 20 cards left and you have the choice between casting Tasha's Hideous Laughter or Glimpse the Unthinkable + hold up Cling to Dust, you might as well take the latter route because opponent is dying next turn anyway. (Against most decks Tasha's hits for ~15 cards.)
- I like to lead with Mesmeric Orb on turn 2 even if it doesn't mill anything, because 1) it self-mills which can be relevant and 2) your other attack spells usually hit for the same amount of damage regardless of when you cast it. However, some decks will have artifact removal (March of Otherworldly Light & Portable Hole especially) so against control & artifact decks it's probably a good idea to wait till some permanents are tapped before playing this.
- I usually cast Ruin Crab on turn 1, especially in an unknown matchup. There aren't many ways to remove it for 1 mana, and blocking a haste creature like Monastery Swiftspear can be relevant. In select matchups or hands you could hold it until you can trigger it, but be sure to plan your hand out when you do so. You don't want to ruin your curve.
- Later in the game it's often advisable to hold a land in case you draw Ruin Crab. However, you usually don't want to hold multiple lands, since you have Lurrus as a mana sink, and since a topdecked Into the Story or Jace can mean you have sudden need for more mana.

Matchups:

There are tons of historic decks so this only covers the most common ones.

Goblins: Skirk Prospector is an important kill target, oddly enough, because it enables an early Muxus and that's an easy way to lose. But if opponent has lords in play with several goblins, kill those too. Goblin Chieftain is also an important kill target because haste is dangerous. Without haste you might get one more turn to do the last points of damage (they have no reach).

Wizards: Slightly favorable to you, but it's very easy to die. The "favor" comes from the fact that they die to Witch's Vengeance and their low curve also means Tasha's often hits for 20 cards against them. Do answer their creatures since if they have to resort to burning you out, there is no way they are faster than you.

Elves: This is about even. You need to draw some answers because they are faster than you otherwise; on the other hand if you answer a few of their key pieces they suddenly stop functioning. Elvish Archdruid is the most important kill target.

Rakdos midrange: This is a bad matchup. Once they get to 4 mana they can just cast Kroxa every turn and kill you. Ideally you have Drown in the Loch or Fatal Push to answer the Kroxa while also casting attack spells at them. Not getting to at least 4 mana is very bad here, because you'll never be able to kill them fast enough otherwise.

Belcher: Just jam unless you have Drown in the Loch and are willing to risk them having Pact of Negation (fortunately Arena's auto-pause tells you if they do). If you can get them below 20 cards in the library they actually can't kill you with one combo.

Bonus section: what about BO3?

Given that Mill is not a good deck, it's kind of pointless to try to build a sideboard for BO3. Your opponents will improve against you, while you won't improve much against most decks, as well.