Spectacular Showdown

Put a double strike counter on target creature, then goad each creature that had a double strike counter put on it this way. (Until your next turn, those creatures attack each combat if able and attack a player other than you if able.)
Overload {4}{R}{R}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Boxing Ring

Whenever a creature you control enters, it fights up to one target creature you don't control with the same mana value.
{T}: Create a Treasure token. Activate only if you control a creature that fought this turn.

Jodah's Codex

Domain — {5}, {T}: Draw a card. This ability costs {1} less to activate for each basic land type among lands you control.

Gravitic Punch

Target creature you control deals damage equal to its power to target player.
Jump-start (You may cast this card from your graveyard by discarding a card in addition to paying its other costs. Then exile this card.)

Nomad Stadium

{T}: Add {W}. Nomad Stadium deals 1 damage to you.
Threshold — {W}, {T}, Sacrifice Nomad Stadium: You gain 4 life. Activate only if seven or more cards are in your graveyard.

Fighting Chance

For each blocking creature, flip a coin. If you win the flip, prevent all combat damage that would be dealt by that creature this turn.

Vikya, Scorching Stalwart

Training (Whenever this creature attacks with another creature with greater power, put a +1/+1 counter on this creature.)
{4}{R}, {Q}, Discard a card: Vikya, Scorching Stalwart deals damage equal to its power to any target. If excess damage was dealt to a creature this way, draw a card. ({Q} is the untap symbol.)

Arena

{3}, {T}: Tap target creature you control and target creature of an opponent's choice they control. Those creatures fight each other. (Each deals damage equal to its power to the other.)

Pit Fight

Target creature you control fights another target creature. (Each deals damage equal to its power to the other.)

Fists of Ironwood

Enchant creature
When Fists of Ironwood enters, create two 1/1 green Saproling creature tokens.
Enchanted creature has trample.

Maarika, Brutal Gladiator

Maarika, Brutal Gladiator must be blocked if able.
As long as it's your turn, Maarika has indestructible.
Whenever Maarika deals damage to a creature, if that creature was dealt excess damage this turn, that creature's controller sacrifices a noncreature, nonland permanent.

Veteran's Armaments

Equipped creature has "Whenever this creature attacks or blocks, it gets +1/+1 until end of turn for each attacking creature."
Whenever a Soldier creature enters, you may attach Veteran's Armaments to it.
Equip {2}

Jodah, the Unifier

Legendary creatures you control get +X/+X, where X is the number of legendary creatures you control.
Whenever you cast a legendary spell from your hand, exile cards from the top of your library until you exile a legendary nonland card with lesser mana value. You may cast that card without paying its mana cost. Put the rest on the bottom of your library in a random order.

Aisha of Sparks and Smoke

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
{R/W}: Aisha of Sparks and Smoke gains first strike until end of turn.
Whenever Aisha deals combat damage, you may cast a sorcery spell from your hand with mana value less than or equal to that damage without paying its mana cost.

Adventurers' Guildhouse

Green legendary creatures you control have "bands with other legendary creatures." (Any legendary creatures can attack in a band as long as at least one has "bands with other legendary creatures." Bands are blocked as a group. If at least two legendary creatures you control, one of which has "bands with other legendary creatures," are blocking or being blocked by the same creature, you divide that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)

Arena Trickster

Whenever you cast your first spell during each opponent's turn, put a +1/+1 counter on Arena Trickster.

Magus of the Arena

Mox Amber

{T}: Add one mana of any color among legendary creatures and planeswalkers you control.

Hajar, Loyal Bodyguard

Sacrifice Hajar, Loyal Bodyguard: Legendary creatures you control get +1/+0 and gain indestructible until end of turn.

Smash to Smithereens

Destroy target artifact. Smash to Smithereens deals 3 damage to that artifact's controller.

Arena Rector

When Arena Rector dies, you may exile it. If you do, search your library for a planeswalker card, put it onto the battlefield, then shuffle.

Unholy Citadel

Black legendary creatures you control have "bands with other legendary creatures." (Any legendary creatures can attack in a band as long as at least one has "bands with other legendary creatures." Bands are blocked as a group. If at least two legendary creatures you control, one of which has "bands with other legendary creatures," are blocking or being blocked by the same creature, you divide that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)

Kick in the Door

Put a +1/+1 counter on target creature. That creature gains haste until end of turn and can't be blocked by Walls this turn. Venture into the dungeon. (Enter the first room or advance to the next room.)

Cathedral of Serra

White legendary creatures you control have "bands with other legendary creatures." (Any legendary creatures can attack in a band as long as at least one has "bands with other legendary creatures." Bands are blocked as a group. If at least two legendary creatures you control, one of which has "bands with other legendary creatures," are blocking or being blocked by the same creature, you divide that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)

Stadium Vendors

When Stadium Vendors enters, choose a player. That player adds two mana of any one color they choose.

Baldin, Century Herdmaster

As long as it's your turn, each creature assigns combat damage equal to its toughness rather than its power.
Whenever Baldin, Century Herdmaster attacks, up to one hundred target creatures each get +0/+X until end of turn, where X is the number of cards in your hand.

Lightning Bolt

Lightning Bolt deals 3 damage to any target.

Fistful of Force

Target creature gets +2/+2 until end of turn. Clash with an opponent. If you win, that creature gets an additional +2/+2 and gains trample until end of turn. (Each clashing player reveals the top card of their library, then puts that card on the top or bottom. A player wins if their card had a higher mana value.)

Seafarer's Quay

Blue legendary creatures you control have "bands with other legendary creatures." (Any legendary creatures can attack in a band as long as at least one has "bands with other legendary creatures." Bands are blocked as a group. If at least two legendary creatures you control, one of which has "bands with other legendary creatures," are blocking or being blocked by the same creature, you divide that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)

Bard Class

(Gain the next level as a sorcery to add its ability.)
Legendary creatures you control enter with an additional +1/+1 counter on them.
{R}{G}: Level 2
Legendary spells you cast cost {R}{G} less to cast. This effect reduces only the amount of colored mana you pay.
{3}{R}{G}: Level 3
Whenever you cast a legendary spell, exile the top two cards of your library. You may play them this turn.

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.

Smash

Fists of Flame

Draw a card. Until end of turn, target creature gains trample and gets +1/+0 for each card you've drawn this turn. (A creature with trample can deal excess combat damage to the player or planeswalker it's attacking.)

Day of Destiny

Legendary creatures you control get +2/+2.

Fight

Aftermath (Cast this spell only from your graveyard. Then exile it.)
Target creature you control fights target creature an opponent controls.

Prepare

Untap target creature. It gets +2/+2 and gains lifelink until end of turn.

Prepare // Fight

Battle Mastery

Enchant creature
Enchanted creature has double strike. (It deals both first-strike and regular combat damage.)

Prizefight

Target creature you control fights target creature you don't control. (Each deals damage equal to its power to the other.)
Create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Fateful Showdown

Fateful Showdown deals damage to any target equal to the number of cards in your hand. Discard all the cards in your hand, then draw that many cards.

Zethi, Arcane Blademaster

Multikicker {W/U}
When Zethi, Arcane Blademaster enters, exile up to X target instant cards from your graveyard, where X is the number of times Zethi was kicked. Put a kick counter on each of them.
Whenever Zethi attacks, copy each exiled card you own with a kick counter on it. You may cast the copies.

Tadeas, Juniper Ascendant

Reach
Tadeas has hexproof unless it's attacking.
Whenever a creature you control with reach attacks, untap it and it can't be blocked by creatures with greater power this combat.
Whenever one or more creatures you control deal combat damage to a player, draw a card.

Phyrexian Arena

At the beginning of your upkeep, you draw a card and you lose 1 life.

Fight Rigging

Hideaway 5 (When this enchantment enters, look at the top five cards of your library, exile one face down, then put the rest on the bottom in a random order.)
At the beginning of combat on your turn, put a +1/+1 counter on target creature you control. Then if you control a creature with power 7 or greater, you may play the exiled card without paying its mana cost.

Tavern Brawler

Commander creatures you own have "At the beginning of your upkeep, exile the top card of your library. This creature gets +X/+0 until end of turn, where X is that card's mana value. You may play that card this turn."

Mountain Stronghold

Red legendary creatures you control have "bands with other legendary creatures." (Any legendary creatures can attack in a band as long as at least one has "bands with other legendary creatures." Bands are blocked as a group. If at least two legendary creatures you control, one of which has "bands with other legendary creatures," are blocking or being blocked by the same creature, you divide that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)

Stronghold Arena

Kicker {G} and/or {W} (You may pay an additional {G} and/or {W} as you cast this spell.)
When Stronghold Arena enters, you gain 3 life for each time it was kicked.
Whenever one or more creatures you control deal combat damage to a player, you may reveal the top card of your library and put it into your hand. If you do, you lose life equal to its mana value.

Arena of the Ancients

Legendary creatures don't untap during their controllers' untap steps.
When Arena of the Ancients enters, tap all legendary creatures.

The Howling Abomination

Haste
The Howling Abomination has trample as long as you've cast three or more spells this turn.
Whenever The Howling Abomination becomes the target of a spell, it gets +2/+2 until end of turn and deals 2 damage to each opponent.

Arena Athlete

Heroic — Whenever you cast a spell that targets Arena Athlete, target creature an opponent controls can't block this turn.

Swift Kick

Target creature you control gets +1/+0 until end of turn. It fights target creature you don't control.

Spinning Wheel Kick

Target creature you control deals damage equal to its power to each of X target creatures and/or planeswalkers.

Obsidian Battle-Axe

Equipped creature gets +2/+1 and has haste.
Whenever a Warrior creature enters, you may attach Obsidian Battle-Axe to it.
Equip {3}

Savage Punch

Target creature you control fights target creature you don't control.
Ferocious — The creature you control gets +2/+2 until end of turn before it fights if you control a creature with power 4 or greater.

Immard, the Stormcleaver

Whenever Immard, the Stormcleaver enters or attacks, put a charge counter on it or remove one from it. When you remove a counter this way, choose one —
• Immard deals 4 damage to any target.
• Immard gains lifelink and indestructible until end of turn.

Undergrowth Stadium

Undergrowth Stadium enters tapped unless you have two or more opponents.
{T}: Add {B} or {G}.

Tournament Grounds

{T}: Add {C}.
{T}: Add {R}, {W}, or {B}. Spend this mana only to cast a Knight or Equipment spell.