You Are Already Dead

Destroy target creature that was dealt damage this turn.
Draw a card.

Disruption Protocol

As an additional cost to cast this spell, tap an untapped artifact you control or pay {1}.
Counter target spell.

Prosperous Thief

Ninjutsu {1}{U} ({1}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever one or more Ninja or Rogue creatures you control deal combat damage to a player, create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Malicious Malfunction

All creatures get -2/-2 until end of turn. If a creature would die this turn, exile it instead.

Inkrise Infiltrator

Flying
{3}{B}: Inkrise Infiltrator gets +2/+2 until end of turn.

Kaito's Pursuit

Target player discards two cards. Ninjas and Rogues you control gain menace until end of turn. (They can't be blocked except by two or more creatures.)

Twisted Embrace

Enchant artifact or creature you control
When Twisted Embrace enters, destroy target creature or planeswalker an opponent controls.
As long as enchanted permanent is a creature, it gets +1/+1.

Spell Pierce

Counter target noncreature spell unless its controller pays {2}.

Silver-Fur Master

Ninjutsu {U}{B} ({U}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Ninjutsu abilities you activate cost {1} less to activate.
Other Ninja and Rogue creatures you control get +1/+1.

Uncharted Haven

This land enters tapped. As it enters, choose a color.
{T}: Add one mana of the chosen color.

Mnemonic Sphere

{1}{U}, Sacrifice Mnemonic Sphere: Draw two cards.
Channel — {U}, Discard Mnemonic Sphere: Draw a card.

Dismal Backwater

This land enters tapped.
When this land enters, you gain 1 life.
{T}: Add {U} or {B}.

Moon-Circuit Hacker

Ninjutsu {U} ({U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever Moon-Circuit Hacker deals combat damage to a player, you may draw a card. If you do, discard a card unless Moon-Circuit Hacker entered this turn.

Swamp

Thousand-Faced Shadow

Ninjutsu {2}{U}{U} ({2}{U}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Flying
When Thousand-Faced Shadow enters from your hand, if it's attacking, create a token that's a copy of another target attacking creature. The token enters tapped and attacking.

Nezumi Prowler

Ninjutsu {1}{B} ({1}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
When Nezumi Prowler enters, target creature you control gains deathtouch and lifelink until end of turn.

Short Circuit

Flash
Enchant artifact or creature
As long as enchanted permanent is a creature, it gets -3/-0 and loses flying.

Dokuchi Silencer

Ninjutsu {1}{B} ({1}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever Dokuchi Silencer deals combat damage to a player, you may discard a creature card. When you do, destroy target creature or planeswalker that player controls.

Essence Capture

Counter target creature spell. Put a +1/+1 counter on up to one target creature you control.

Lethal Exploit

Target creature gets -2/-2 until end of turn. It gets an additional -1/-1 until end of turn for each modified creature you controlled as you cast this spell. (Equipment, Auras you control, and counters are modifications.)

Planar Incision

Exile target artifact or creature, then return it to the battlefield under its owner's control with a +1/+1 counter on it.

Island

Return to Action

Until end of turn, target creature gets +1/+0 and gains lifelink and "When this creature dies, return it to the battlefield tapped under its owner's control."

Ninja's Kunai

Equipped creature has "{1}, {T}, Sacrifice Ninja's Kunai: Ninja's Kunai deals 3 damage to any target."
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)