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Opposition Agent

Flash
You control your opponents while they're searching their libraries.
While an opponent is searching their library, they exile each card they find. You may play those cards for as long as they remain exiled, and you may spend mana as though it were mana of any color to cast them.

Boseiju, Who Shelters All

Boseiju, Who Shelters All enters the battlefield tapped.
{T}, Pay 2 life: Add {C}. If that mana is spent on an instant or sorcery spell, that spell can't be countered.

Sickening Dreams

As an additional cost to cast this spell, discard X cards.
Sickening Dreams deals X damage to each creature and each player.

Maralen of the Mornsong

Players can't draw cards.
At the beginning of each player's draw step, that player loses 3 life, searches their library for a card, puts it into their hand, then shuffles.

Sol Ring

Memnite

Profane Tutor

Suspend 2—{1}{B} (Rather than cast this card from your hand, pay {1}{B} and exile it with two time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost.)
Search your library for a card, put that card into your hand, then shuffle.

Phyrexian Walker

Skirge Familiar

Ornithopter

Blood Pet

Urza's Saga

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — Urza's Saga gains "{T}: Add {C}."
II — Urza's Saga gains "{2}, {T}: Create a 0/0 colorless Construct artifact creature token with 'This creature gets +1/+1 for each artifact you control.'"
III — Search your library for an artifact card with mana cost {0} or {1}, put it onto the battlefield, then shuffle.

Dark Sphere

{T}, Sacrifice Dark Sphere: The next time a source of your choice would deal damage to you this turn, prevent half that damage, rounded down.

Chrome Mox

Imprint — When Chrome Mox enters the battlefield, you may exile a nonartifact, nonland card from your hand.
{T}: Add one mana of any of the exiled card's colors.

Dark Ritual

Glacial Chasm

Cumulative upkeep—Pay 2 life. (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
When Glacial Chasm enters the battlefield, sacrifice a land.
Creatures you control can't attack.
Prevent all damage that would be dealt to you.

Reliquary Tower

You have no maximum hand size.
{T}: Add {C}.

Gix, Yawgmoth Praetor

Whenever a creature deals combat damage to one of your opponents, its controller may pay 1 life. If they do, they draw a card.
{4}{B}{B}{B}, Discard X cards: Exile the top X cards of target opponent's library. You may play lands and cast spells from among cards exiled this way without paying their mana costs.

Culling the Weak

As an additional cost to cast this spell, sacrifice a creature.
Add {B}{B}{B}{B}.

Shield Sphere

Defender
Whenever Shield Sphere blocks, put a -0/-1 counter on it.

Elixir of Immortality

{2}, {T}: You gain 5 life. Shuffle Elixir of Immortality and your graveyard into their owner's library.

Swamp

Cabal Ritual

Add {B}{B}{B}.
Threshold — Add {B}{B}{B}{B}{B} instead if seven or more cards are in your graveyard.

Imp's Mischief

Change the target of target spell with a single target. You lose life equal to that spell's mana value.

Rain of Filth

Until end of turn, lands you control gain "Sacrifice this land: Add {B}."

Jeweled Lotus

{T}, Sacrifice Jeweled Lotus: Add three mana of any one color. Spend this mana only to cast your commander.

Diabolic Intent

As an additional cost to cast this spell, sacrifice a creature.
Search your library for a card, put that card into your hand, then shuffle.

Mirrored Lotus

Reflect {0} (As this enters the battlefield, each opponent may pay {0}. When they do, they create a token copy of this except it lacks this ability.)
{T}, Exile Mirrored Lotus: Add three mana of any one color.

Demonic Tutor

Search your library for a card, put that card into your hand, then shuffle.

Pact Weapon

As long as Pact Weapon is attached to a creature, you don't lose the game for having 0 or less life.
Whenever equipped creature attacks, draw a card and reveal it. The creature gets +X/+X until end of turn and you lose X life, where X is that card's mana value.
Equip—Discard a card.

Ad Nauseam

Reveal the top card of your library and put that card into your hand. You lose life equal to its mana value. You may repeat this process any number of times.

Mana Crypt

At the beginning of your upkeep, flip a coin. If you lose the flip, Mana Crypt deals 3 damage to you.
{T}: Add {C}{C}.

Dark Deal

Each player discards all the cards in their hand, then draws that many cards minus one.

Vampiric Tutor

Search your library for a card, then shuffle and put that card on top. You lose 2 life.