Arm with Aether

Until end of turn, creatures you control gain "Whenever this creature deals damage to an opponent, you may return target creature that player controls to its owner's hand."

Hidden Strings

You may tap or untap target permanent, then you may tap or untap another target permanent.
Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)

Cephalid Constable

Whenever Cephalid Constable deals combat damage to a player, return up to that many target permanents that player controls to their owner's hand.

Endless Evil

Enchant creature you control
At the beginning of your upkeep, create a token that's a copy of enchanted creature, except the token is 1/1.
When enchanted creature dies, if that creature was a Horror, return Endless Evil to its owner's hand.

Bident of Thassa

Whenever a creature you control deals combat damage to a player, you may draw a card.
{1}{U}, {T}: Creatures your opponents control attack this turn if able.

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 the battlefield from your hand, if it's attacking, create a token that's a copy of another target attacking creature. The token enters the battlefield tapped and attacking.

Sigil of Sleep

Enchant creature
Whenever enchanted creature deals damage to a player, return target creature that player controls to its owner's hand.

Prying Blade

Equipped creature gets +1/+0.
Whenever equipped creature deals combat damage to a player, create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Feywild Visitor

Commander creatures you own have "Whenever one or more nontoken creatures you control deal combat damage to a player, you create a 1/1 blue Faerie Dragon creature token with flying."

Dreamshackle Geist

Flying
At the beginning of combat on your turn, choose up to one —
• Tap target creature.
• Target creature doesn't untap during its controller's next untap step.

Coastal Piracy

Whenever a creature you control deals combat damage to an opponent, you may draw a card.

Loxodon Warhammer

Equipped creature gets +3/+0 and has trample and lifelink.
Equip {3}

Neurok Commando

Shroud (This creature can't be the target of spells or abilities.)
Whenever Neurok Commando deals combat damage to a player, you may draw a card.

Corrupted Resolve

Counter target spell if its controller is poisoned.

Cloak and Dagger

Equipped creature gets +2/+0 and has shroud. (It can't be the target of spells or abilities.)
Whenever a Rogue creature enters the battlefield, you may attach Cloak and Dagger to it.
Equip {3}

Mist-Syndicate Naga

Ninjutsu {2}{U} ({2}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever Mist-Syndicate Naga deals combat damage to a player, create a token that's a copy of Mist-Syndicate Naga.

Catlike Curiosity

Enchant creature
Enchanted creature has "Whenever this creature deals combat damage to a player, draw a card."
If Catlike Curiosity would be put into a graveyard from anywhere, exile it instead.

Mischievous Catgeist

Whenever Mischievous Catgeist deals combat damage to a player, draw a card.
Disturb {2}{U} (You may cast this card from your graveyard transformed for its disturb cost.)

Mischievous Catgeist // Catlike Curiosity

Notorious Throng

Prowl {5}{U} (You may cast this for its prowl cost if you dealt combat damage to a player this turn with a Rogue.)
Create X 1/1 black Faerie Rogue creature tokens with flying, where X is the damage dealt to your opponents this turn. If this spell's prowl cost was paid, take an extra turn after this one.

Necropede

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
When Necropede dies, you may put a -1/-1 counter on target creature.

Guildpact Informant

Flying
Whenever Guildpact Informant deals combat damage to a player or planeswalker, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Dreadful as the Storm

Target creature has base power and toughness 5/5 until end of turn. The Ring tempts you.

Island

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.")

Thought Vessel

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

Ghostly Pilferer

Whenever Ghostly Pilferer becomes untapped, you may pay {2}. If you do, draw a card.
Whenever an opponent casts a spell from anywhere other than their hand, draw a card.
Discard a card: Ghostly Pilferer can't be blocked this turn.

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 the battlefield this turn.

Thrummingbird

Flying
Whenever Thrummingbird deals combat damage to a player, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Sol Ring

Dune Mover

Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
When Dune Mover enters the battlefield, you may search your library for a basic land card, reveal it, then shuffle and put that card on top.

Blighted Agent

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Blighted Agent can't be blocked.

Cephalid Facetaker

Cephalid Facetaker can't be blocked.
At the beginning of combat on your turn, you may have Cephalid Facetaker become a copy of another target creature until end of turn, except it's 1/4 and has "This creature can't be blocked."

Viral Drake

Flying
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
{3}{U}: Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Myriad Landscape

Myriad Landscape enters the battlefield tapped.
{T}: Add {C}.
{2}, {T}, Sacrifice Myriad Landscape: Search your library for up to two basic land cards that share a land type, put them onto the battlefield tapped, then shuffle.

Quietus Spike

Equipped creature has deathtouch.
Whenever equipped creature deals combat damage to a player, that player loses half their life, rounded up.
Equip {3}

Prologue to Phyresis

Each opponent gets a poison counter.
Draw a card.

Scroll Thief

Whenever Scroll Thief deals combat damage to a player, draw a card.

Tandem Lookout

Soulbond (You may pair this creature with another unpaired creature when either enters the battlefield. They remain paired for as long as you control both of them.)
As long as Tandem Lookout is paired with another creature, each of those creatures has "Whenever this creature deals damage to an opponent, draw a card."

Library Larcenist

Whenever Library Larcenist attacks, draw a card.

Last Thoughts

Draw a card.
Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)

Mind Stone

{T}: Add {C}.
{1}, {T}, Sacrifice Mind Stone: Draw a card.

Fuel for the Cause

Counter target spell, then proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Phyrexian Digester

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Surrakar Spellblade

Whenever you cast an instant or sorcery spell, you may put a charge counter on Surrakar Spellblade.
Whenever Surrakar Spellblade deals combat damage to a player, you may draw X cards, where X is the number of charge counters on it.

Master of Waves

Protection from red
Elemental creatures you control get +1/+1.
When Master of Waves enters the battlefield, create a number of 1/0 blue Elemental creature tokens equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

Strixhaven Stadium

{T}: Add {C}. Put a point counter on Strixhaven Stadium.
Whenever a creature deals combat damage to you, remove a point counter from Strixhaven Stadium.
Whenever a creature you control deals combat damage to an opponent, put a point counter on Strixhaven Stadium. Then if it has ten or more point counters on it, remove them all and that player loses the game.

Foil

You may discard an Island card and another card rather than pay this spell's mana cost.
Counter target spell.

Flux Channeler

Whenever you cast a noncreature spell, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Experimental Augury

Look at the top three cards of your library. Put one of them into your hand and the rest on the bottom of your library in any order. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Arcane Signet

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

Serum Snare

Return target nonland permanent to its owner's hand. If that permanent had mana value 3 or less, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Riptide Entrancer

Whenever Riptide Entrancer deals combat damage to a player, you may sacrifice it. If you do, gain control of target creature that player controls. (This effect lasts indefinitely.)
Morph {U}{U} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)

Zhalfirin Shapecraft

Target creature has base power and toughness 4/3 until end of turn.
Draw a card.

Contentious Plan

Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
Draw a card.

Glistening Sphere

Glistening Sphere enters the battlefield tapped.
When Glistening Sphere enters the battlefield, proliferate.
{T}: Add one mana of any color.
Corrupted — {T}: Add three mana of any one color. Activate only if an opponent has three or more poison counters.

Rewind

Counter target spell. Untap up to four lands.

Mistblade Shinobi

Ninjutsu {U} ({U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever Mistblade Shinobi deals combat damage to a player, you may return target creature that player controls to its owner's hand.

Minn, Wily Illusionist

Whenever you draw your second card each turn, create a 1/1 blue Illusion creature token with "This creature gets +1/+0 for each other Illusion you control."
Whenever an Illusion you control dies, you may put a permanent card with mana value less than or equal to that creature's power from your hand onto the battlefield.

Stolen Identity

Create a token that's a copy of target artifact or creature.
Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)

Myr Convert

Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
{T}, Pay 2 life: Add one mana of any color.

Teferi's Veil

Whenever a creature you control attacks, it phases out at end of combat. (While it's phased out, it's treated as though it doesn't exist. It phases in before you untap during your next untap step.)

Ichorclaw Myr

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Whenever Ichorclaw Myr becomes blocked, it gets +2/+2 until end of turn.

Ghastly Haunting

Enchant creature
You control enchanted creature.

Soul Seizer

Flying
When Soul Seizer deals combat damage to a player, you may transform it. If you do, attach it to target creature that player controls.

Soul Seizer // Ghastly Haunting

Tetsuko Umezawa, Fugitive

Creatures you control with power or toughness 1 or less can't be blocked.

Eternal of Harsh Truths

Afflict 2 (Whenever this creature becomes blocked, defending player loses 2 life.)
Whenever Eternal of Harsh Truths attacks and isn't blocked, draw a card.

Plague Myr

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
{T}: Add {C}.

Reconnaissance Mission

Whenever a creature you control deals combat damage to a player, you may draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)