Counterspell

Crumbling Necropolis

Crumbling Necropolis enters tapped.
{T}: Add {U}, {B}, or {R}.

Corporeal Projection

Target creature you control gains myriad until end of turn. (Whenever it attacks, for each opponent other than defending player, you may create a token copy that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)
Overload {3}{U}{U}{R}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Talisman of Dominance

{T}: Add {C}.
{T}: Add {U} or {B}. Talisman of Dominance deals 1 damage to you.

Mountain

Sol Ring

Kaito, Dancing Shadow

Whenever one or more creatures you control deal combat damage to a player, you may return one of them to its owner's hand. If you do, you may activate loyalty abilities of Kaito twice this turn rather than only once.
+1: Up to one target creature can't attack or block until your next turn.
0: Draw a card.
−2: Create a 2/2 colorless Drone artifact creature token with deathtouch and "When this creature leaves the battlefield, each opponent loses 2 life and you gain 2 life."

Exotic Orchard

{T}: Add one mana of any color that a land an opponent controls could produce.

Rakdos Signet

Goro-Goro and Satoru

Whenever one or more creatures you control that entered this turn deal combat damage to a player, create a 5/5 red Dragon Spirit creature token with flying.
{1}{R}: Creatures you control gain haste until end of turn.

Command Tower

{T}: Add one mana of any color in your commander's color identity.

Reliquary Tower

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

Izzet Signet

Dimir Signet

Scatter Ray

Counter target artifact or creature spell unless its controller pays {4}.

Swamp

Island

Wayfarer's Bauble

{2}, {T}, Sacrifice Wayfarer's Bauble: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Talisman of Indulgence

{T}: Add {C}.
{T}: Add {B} or {R}. Talisman of Indulgence deals 1 damage to you.

Song of Totentanz

Create X 1/1 black Rat creature tokens with "This creature can't block." Creatures you control gain haste until end of turn.

Bojuka Bog

Bojuka Bog enters tapped.
When Bojuka Bog enters, exile target player's graveyard.
{T}: Add {B}.

Path of Ancestry

Path of Ancestry enters tapped.
{T}: Add one mana of any color in your commander's color identity. When that mana is spent to cast a creature spell that shares a creature type with your commander, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Negate

Arcane Signet

Bedevil

Destroy target artifact, creature, or planeswalker.

Terminate

Destroy target creature. It can't be regenerated.

Blasphemous Act

This spell costs {1} less to cast for each creature on the battlefield.
Blasphemous Act deals 13 damage to each creature.

Feed the Swarm

Destroy target creature or enchantment an opponent controls. You lose life equal to that permanent's mana value.

Chain Reaction

Chain Reaction deals X damage to each creature, where X is the number of creatures on the battlefield.

Talisman of Creativity

{T}: Add {C}.
{T}: Add {U} or {R}. Talisman of Creativity deals 1 damage to you.

Chaos Warp

The owner of target permanent shuffles it into their library, then reveals the top card of their library. If it's a permanent card, they put it onto the battlefield.

Sulfur Falls

Sulfur Falls enters tapped unless you control an Island or a Mountain.
{T}: Add {U} or {R}.

Sunken Hollow

({T}: Add {U} or {B}.)
Sunken Hollow enters tapped unless you control two or more basic lands.

Brainstorm

Draw three cards, then put two cards from your hand on top of your library in any order.