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.

Karakas

{T}: Add {W}.
{T}: Return target legendary creature to its owner's hand.

Ancient Tomb

{T}: Add {C}{C}. This land deals 2 damage to you.

Eliminate

Destroy target creature or planeswalker with mana value 3 or less.

Dark Confidant

At the beginning of your upkeep, reveal the top card of your library and put that card into your hand. You lose life equal to its mana value.

Helm of Obedience

{X}, {T}: Target opponent mills a card, then repeats this process until a creature card or X cards have been put into their graveyard this way, whichever comes first. If one or more creature cards were put into that graveyard this way, sacrifice this artifact and put one of them onto the battlefield under your control. X can't be 0.

Expedition Map

{2}, {T}, Sacrifice this artifact: Search your library for a land card, reveal it, put it into your hand, then shuffle.

Dark Depths

Dark Depths enters with ten ice counters on it.
{3}: Remove an ice counter from Dark Depths.
When Dark Depths has no ice counters on it, sacrifice it. If you do, create Marit Lage, a legendary 20/20 black Avatar creature token with flying and indestructible.

Sudden Edict

Split second (As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)
Target player sacrifices a creature of their choice.

Massacre

If an opponent controls a Plains and you control a Swamp, you may cast this spell without paying its mana cost.
All creatures get -2/-2 until end of turn.

Karn, Scion of Urza

+1: Reveal the top two cards of your library. An opponent chooses one of them. Put that card into your hand and exile the other with a silver counter on it.
−1: Put a card you own with a silver counter on it from exile into your hand.
−2: Create a 0/0 colorless Construct artifact creature token with "This token gets +1/+1 for each artifact you control."

Rite of Consumption

As an additional cost to cast this spell, sacrifice a creature.
Rite of Consumption deals damage equal to the sacrificed creature's power to target player or planeswalker. You gain life equal to the damage dealt this way.

Swamp

Vampire Hexmage

First strike
Sacrifice this creature: Remove all counters from target permanent.

Dauthi Voidwalker

Shadow (This creature can block or be blocked by only creatures with shadow.)
If a card would be put into an opponent's graveyard from anywhere, instead exile it with a void counter on it.
{T}, Sacrifice this creature: Choose an exiled card an opponent owns with a void counter on it. You may play it this turn without paying its mana cost.

Shadowspear

Equipped creature gets +1/+1 and has trample and lifelink.
{1}: Permanents your opponents control lose hexproof and indestructible until end of turn.
Equip {2}

Plague Engineer

Deathtouch
As this creature enters, choose a creature type.
Creatures of the chosen type your opponents control get -1/-1.

Pithing Needle

As this artifact enters, choose a card name.
Activated abilities of sources with the chosen name can't be activated unless they're mana abilities.

Duress

Target opponent reveals their hand. You choose a noncreature, nonland card from it. That player discards that card.

Nihil Spellbomb

{T}, Sacrifice this artifact: Exile target player's graveyard.
When this artifact is put into a graveyard from the battlefield, you may pay {B}. If you do, draw a card.

Thoughtseize

Target player reveals their hand. You choose a nonland card from it. That player discards that card. You lose 2 life.

Leyline of the Void

If this card is in your opening hand, you may begin the game with it on the battlefield.
If a card would be put into an opponent's graveyard from anywhere, exile it instead.

Dark Ritual

Urza's Saga

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — This Saga gains "{T}: Add {C}."
II — This Saga gains "{2}, {T}: Create a 0/0 colorless Construct artifact creature token with 'This token 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.

Engineered Explosives

Sunburst (This enters with a charge counter on it for each color of mana spent to cast it.)
{2}, Sacrifice this artifact: Destroy each nonland permanent with mana value equal to the number of charge counters on this artifact.

Urborg, Tomb of Yawgmoth

Each land is a Swamp in addition to its other land types.

Thespian's Stage

{T}: Add {C}.
{2}, {T}: This land becomes a copy of target land, except it has this ability.

Bojuka Bog

This land enters tapped.
When this land enters, exile target player's graveyard.
{T}: Add {B}.