Saruman of Many Colors

Ward—Discard an enchantment, instant, or sorcery card.
Whenever you cast your second spell each turn, each opponent mills two cards. When one or more cards are milled this way, exile target enchantment, instant, or sorcery card with equal or lesser mana value than that spell from an opponent's graveyard. Copy the exiled card. You may cast the copy without paying its mana cost.

Chronic Flooding

Enchant land
Whenever enchanted land becomes tapped, its controller mills three cards.

Countermand

Counter target spell. Its controller mills four cards.

Enigma Eidolon

{U}, Sacrifice Enigma Eidolon: Target player mills three cards.
Whenever you cast a multicolored spell, you may return Enigma Eidolon from your graveyard to your hand.

King Narfi's Betrayal

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — Each player mills four cards. Then you may exile a creature or planeswalker card from each graveyard.
II, III — Until end of turn, you may cast spells from among cards exiled with King Narfi's Betrayal, and you may spend mana as though it were mana of any color to cast those spells.

Go-Shintai of Lost Wisdom

Flying
At the beginning of your end step, you may pay {1}. When you do, target player mills X cards, where X is the number of Shrines you control. (To mill a card, a player puts the top card of their library into their graveyard.)

Geralf's Mindcrusher

When Geralf's Mindcrusher enters, target player mills five cards.
Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)

Font of Progress

Font of Progress enters with two oil counters on it.
{3}, {T}: Target player mills X cards, where X is the number of oil counters on Font of Progress.

Extractor Demon

Flying
Whenever another creature leaves the battlefield, you may have target player mill two cards.
Unearth {2}{B} ({2}{B}: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the next end step or if it would leave the battlefield. Unearth only as a sorcery.)

Didn't Say Please

Counter target spell. Its controller mills three cards.

Homarid Explorer

When Homarid Explorer enters, target player mills four cards.

Doomskar Oracle

Whenever you cast your second spell each turn, you gain 2 life.
Foretell {W} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Extract from Darkness

Each player mills two cards. Then you put a creature card from a graveyard onto the battlefield under your control.

Drownyard Amalgam

When Drownyard Amalgam enters, target player mills three cards. (They put the top three cards of their library into their graveyard.)
{2}{U}: Drownyard Amalgam can't be blocked this turn.

Cloudhoof Kirin

Flying
Whenever you cast a Spirit or Arcane spell, you may have target player mill X cards, where X is that spell's mana value.

Dampen Thought

Target player mills four cards.
Splice onto Arcane {1}{U} (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)

Memory Sluice

Target player mills four cards.
Conspire (As you cast this spell, you may tap two untapped creatures you control that share a color with it. When you do, copy it and you may choose a new target for the copy.)

Chilling Trap

Target creature gets -4/-0 until end of turn. If you control a Wizard, draw a card.

Pelargir Survivor

{T}: Add one mana of any color. Spend this mana only to cast an instant or sorcery spell.
{5}{U}, {T}: Target player mills three cards. (They put the top three cards of their library into their graveyard.)

Deal Gone Bad

Target creature gets -3/-3 until end of turn. Target player mills three cards. (They put the top three cards of their library into their graveyard.)

Swamp

Krydle of Baldur's Gate

Whenever Krydle of Baldur's Gate deals combat damage to a player, that player loses 1 life and mills a card, then you gain 1 life and scry 1.
Whenever you attack, you may pay {2}. If you do, target creature can't be blocked this turn.

The Mouth of Sauron

When The Mouth of Sauron enters, target player mills three cards. Then amass Orcs X, where X is the number of instant and sorcery cards in that player's graveyard. (Put X +1/+1 counters on an Army you control. It's also an Orc. If you don't control an Army, create a 0/0 black Orc Army creature token first.)

Compelling Argument

Target player mills five cards.
Cycling {U} ({U}, Discard this card: Draw a card.)

Demogorgon's Clutches

Target opponent discards two cards, mills two cards, and loses 2 life. (To mill a card, a player puts the top card of their library into their graveyard.)

Clarion Spirit

Whenever you cast your second spell each turn, create a 1/1 white Spirit creature token with flying.

Surrounded by Orcs

Amass Orcs 3, then target player mills X cards, where X is the amassed Army's power. (To amass Orcs 3, put three +1/+1 counters on an Army you control. It's also an Orc. If you don't control an Army, create a 0/0 black Orc Army creature token first.)

Horrifying Revelation

Target player discards a card, then mills a card.

Codex Shredder

{T}: Target player mills a card. (They put the top card of their library into their graveyard.)
{5}, {T}, Sacrifice Codex Shredder: Return target card from your graveyard to your hand.

Crosstown Courier

Whenever Crosstown Courier deals combat damage to a player, that player mills that many cards.

Monk of the Open Hand

Flurry of Blows — Whenever you cast your second spell each turn, put a +1/+1 counter on Monk of the Open Hand.

Captain N'ghathrod

Horrors you control have menace.
Whenever a Horror you control deals combat damage to a player, that player mills that many cards.
At the beginning of your end step, choose target artifact or creature card in an opponent's graveyard that was put there from their library this turn. Put it onto the battlefield under your control.

Eye Collector

Flying
Whenever Eye Collector deals combat damage to a player, each player mills a card. (Each player puts the top card of their library into their graveyard.)

Cut Your Losses

Casualty 2 (As you cast this spell, you may sacrifice a creature with power 2 or greater. When you do, copy this spell and you may choose a new target for the copy.)
Target player mills half their library, rounded down.

Plains

Duskmantle, House of Shadow

{T}: Add {C}.
{U}{B}, {T}: Target player mills a card.

Glacial Grasp

Tap target creature. Its controller mills two cards. That creature doesn't untap during its controller's next untap step. (They put the top two cards of their library into their graveyard.)
Draw a card.

Drowned Secrets

Whenever you cast a blue spell, target player mills two cards.

Fleeting Memories

When Fleeting Memories enters, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Whenever you sacrifice a Clue, target player mills three cards.

Folio of Fancies

Players have no maximum hand size.
{X}{X}, {T}: Each player draws X cards.
{2}{U}, {T}: Each opponent mills cards equal to the number of cards in their hand.

Dread Summons

Each player mills X cards. For each creature card put into a graveyard this way, you create a tapped 2/2 black Zombie creature token. (To mill a card, a player puts the top card of their library into their graveyard.)

Jace's Erasure

Whenever you draw a card, you may have target player mill a card.

Gnawing Vermin

When Gnawing Vermin enters, target player mills two cards.
When Gnawing Vermin dies, target creature you don't control gets -1/-1 until end of turn.

Rivendell

Rivendell enters tapped unless you control a legendary creature.
{T}: Add {U}.
{1}{U}, {T}: Scry 2. Activate only if you control a legendary creature.

Curse of the Bloody Tome

Enchant player
At the beginning of enchanted player's upkeep, that player mills two cards.

Belltower Sphinx

Flying
Whenever a source deals damage to Belltower Sphinx, that source's controller mills that many cards.

Fractured Sanity

Each opponent mills fourteen cards.
Cycling {1}{U} ({1}{U}, Discard this card: Draw a card.)
When you cycle Fractured Sanity, each opponent mills four cards.

Island

Fleet Swallower

Whenever Fleet Swallower attacks, target player mills half their library, rounded up.

So Tiny

Flash
Enchant creature
Enchanted creature gets -2/-0. It gets -6/-0 instead as long as its controller has seven or more cards in their graveyard.

Decimator Web

{4}, {T}: Target opponent loses 2 life, gets a poison counter, then mills six cards.

Coerced Confession

Target player mills four cards. You draw a card for each creature card put into their graveyard this way.

Flint Golem

Whenever Flint Golem becomes blocked, defending player mills three cards.

Dream Twist

Target player mills three cards.
Flashback {1}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Dreadwaters

Target player mills X cards, where X is the number of lands you control.

Disturbing Conversion

Flash
Enchant creature
When Disturbing Conversion enters, each player mills two cards.
Enchanted creature gets -X/-0, where X is the number of cards in its controller's graveyard.

Enter the God-Eternals

Enter the God-Eternals deals 4 damage to target creature and you gain life equal to the damage dealt this way. Target player mills four cards. Amass Zombies 4. (Put four +1/+1 counters on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)

Cathartic Adept

Koma's Faithful

Lifelink (Damage dealt by this creature also causes you to gain that much life.)
When Koma's Faithful dies, each player mills three cards. (They each put the top three cards of their library into their graveyard.)

Duskmantle Guildmage

{1}{U}{B}: Whenever a card is put into an opponent's graveyard from anywhere this turn, that player loses 1 life.
{2}{U}{B}: Target player mills two cards.

Court of Cunning

When Court of Cunning enters, you become the monarch.
At the beginning of your upkeep, any number of target players each mill two cards. If you're the monarch, each of those players mills ten cards instead. (To mill a card, a player puts the top card of their library into their graveyard.)

Cyclone Summoner

When Cyclone Summoner enters, if you cast it from your hand, return all permanents to their owners' hands except for Giants, Wizards, and lands.

Atrocious Experiment

Target player mills two cards, draws two cards, and loses 2 life. (To mill a card, a player puts the top card of their library into their graveyard.)