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.

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.

Defile

Target creature gets -1/-1 until end of turn for each Swamp you control.

Ashnod's Altar

Dark Ritual

Boseiju, Who Shelters All

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

Lotus Petal

{T}, Sacrifice Lotus Petal: Add one mana of any color.

Torment of Hailfire

Repeat the following process X times. Each opponent loses 3 life unless that player sacrifices a nonland permanent or discards a card.

Bubbling Muck

Until end of turn, whenever a player taps a Swamp for mana, that player adds an additional {B}.

Everflowing Chalice

Multikicker {2} (You may pay an additional {2} any number of times as you cast this spell.)
Everflowing Chalice enters with a charge counter on it for each time it was kicked.
{T}: Add {C} for each charge counter on Everflowing Chalice.

Zombie Infestation

Discard two cards: Create a 2/2 black Zombie creature token.

Culling the Weak

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

Sickening Dreams

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

Elixir of Immortality

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

Stunning Reversal

The next time you would lose the game this turn, instead draw seven cards and your life total becomes 1.
Exile Stunning Reversal.

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.

Snow-Covered Swamp

Spellbook

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.