Modern Horizons 3’s spoiler season has been accelerated significantly by a collection of leaks. Even today, a flurry of exciting leaks hit the internet, but one leak seemed to shine above them all.
Sylvan Library

As you may already know, a prevailing theme with Modern Horizons 3 appears to be the reinvention of old staples with a new coat of paint. A lot of the time, these are being powered down a little to make them integrate into Modern properly. While this is still weaker than Sylvan Library, it enables a different kind of strategy.
Of course, since this is a leak, there is a chance the contents might be fake. As a result, consider this your spoiler warning.
Ripples of Undeath

The Sylvan Library variant that players are going crazy over is Ripples of Undeath. Here’s what it does:
Mana Value 1B Enchantment
At the beginning of your precombat main phase, mill three cards. Then you may pay 1 mana and 3 life. If you do, put a card from among those cards into your hand.
Instead of drawing three cards and keeping some number of them, Ripples of Undeath mills three cards. Sadly, you can only choose one of these cards to add to your hand, and you need to pay one mana and three life to do it. In terms of acquiring card advantage, this is worse than Sylvan Library, but the card is still incredibly efficient.
Obviously, decks that want to mill cards are the ones that want Ripples of Undeath the most. Even if you decide not to add cards to your hand, you’re still milling three cards and adding to your gameplan. This can fuel a ton of different things in the Modern format, including Delirium, Delve payoffs for
Murktide Regent

Flying
This creature enters with a +1/+1 counter on it for each instant and sorcery card exiled with it.
Whenever an instant or sorcery card leaves your graveyard, put a +1/+1 counter on this creature.
, Dredge/Crabvine strategies and even
Living End

Each player exiles all creature cards from their graveyard, then sacrifices all creatures they control, then puts all cards they exiled this way onto the battlefield.
variants that utilize
Bloodbraid Marauder

Delirium — This spell has cascade as long as there are four or more card types among cards in your graveyard. (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.)
as the Cascade weapon of choice.
Another example of a deck that definitely wants Ripples of Undeath is
Goryo’s Vengeance

Splice onto Arcane {2}{B} (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.)
. Not only does this get your reanimation payoffs like
Griselbrand

Pay 7 life: Draw seven cards.
and
Atraxa, Grand Unifier

When Atraxa enters, reveal the top ten cards of your library. For each card type, you may put a card of that type from among the revealed cards into your hand. Put the rest on the bottom of your library in a random order. (Artifact, battle, creature, enchantment, instant, land, planeswalker, and sorcery are card types.)
into the bin, you can also find your Goryo’s Vengeance and add it to your hand. Otherwise, you can mill Flashback cards like
Faithful Mending

Flashback {1}{W}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)
for additional value.
Death’s Shadow

All in all, a ton of different archetypes serve to gain something from this card. Even Rakdos Scam could be interested in the card advantage implications of Ripples of Undeath. It can act as a pseudo sideboard option for Living End, able to dump your Dauthi Voidwalkers into your grave.
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Ripples of Undeath in Commander

Honestly, Ripples of Undeath is likely to be an auto-include in any black deck that cares about their grave. One mana and three life is a much smaller cost in a format with abundant resources, and it advances your gameplan otherwise. For two mana, this repetitive effect is absolutely incredible value.
Muldrotha, the Gravetide seems like an obvious home for Ripples of Undeath. Not only does this enable Muldrotha’s gameplan, but Muldrotha can even recast this card from the grave if it gets destroyed.
Another Commander that seems great with this card is
Raul, Troubleshooter

{T}: Each player mills a card. (They each put the top card of their library into their graveyard.)
. Raul allows Ripples of Undeath to do double duty, gaining you a card and milling cards for Raul to cast.
The Wise Mothman

Whenever The Wise Mothman enters or attacks, each player gets a rad counter.
Whenever one or more nonland cards are milled, put a +1/+1 counter on each of up to X target creatures, where X is the number of nonland cards milled this way.
benefits from Ripples of Undeath, allowing this to be a fantastic upgrade option for the Mutant Menace precon. Nonland mills will pump up your board, and Ripples of Undeath will generate card advantage. This card has similar synergies with
The Master, Transcendent

{T}: Put target creature card in a graveyard that was milled this turn onto the battlefield under your control. It's a green Mutant with base power and toughness 3/3. (It loses its other colors and creature types.)
,
Sidisi, Brood Tyrant

Whenever one or more creature cards are put into your graveyard from your library, create a 2/2 black Zombie creature token.
and
The Mycotyrant

The Mycotyrant's power and toughness are each equal to the number of creatures you control that are Fungi and/or Saprolings.
At the beginning of your end step, create X 1/1 black Fungus creature tokens with "This token can't block," where X is the number of times you descended this turn. (You descend each time a permanent card is put into your graveyard from anywhere.)
. All of your permanent and creature mills from Ripples of Undeath will generate additional card advantage.
This is Going to be Expensive (If Real)
Ripples of Undeath has some serious potential. Not only are Modern players going to be interested in this, Commander players will want some copies as well. Even some older formats could be interested in giving this card a try.
Demanding interest from different playerbases, Ripples of Undeath is ripe to cause a surge in demand. As a result, if this card is real, we would not be surprised if it had some secondary market value.
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