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Commander
At the beginning of combat on your turn, each Merfolk creature you control explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on the exploring creature, then put the card back or put it into your graveyard.) Whenever Hakbal of the Surging Soul attacks, you may put a land card from your hand onto the battlefield. If you don't, draw a card.
Counters
When Court of Garenbrig enters the battlefield, you become the monarch. At the beginning of your upkeep, distribute two +1/+1 counters among up to two target creatures. Then if you're the monarch, double the number of +1/+1 counters on each creature you control.
If one or more +1/+1 counters would be put on an artifact or creature you control, that many plus one +1/+1 counters are put on it instead. {1}{G}, {T}: Put a +1/+1 counter on target artifact or creature you control. Activate only as a sorcery. Cycling {2} ({2}, Discard this card: Draw a card.)
Creature
At the beginning of combat on your turn, Deepfathom Echo explores. Then you may have it become a copy of another creature you control until end of turn. (To have this creature explore, reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)
Ward {2} As Roaming Throne enters the battlefield, choose a creature type. Roaming Throne is the chosen type in addition to its other types. If a triggered ability of another creature you control of the chosen type triggers, it triggers an additional time.
You may have Sakashima the Impostor enter the battlefield as a copy of any creature on the battlefield, except its name is Sakashima the Impostor, it's legendary in addition to its other types, and it has "{2}{U}{U}: Return this creature to its owner's hand at the beginning of the next end step."
You may have Spark Double enter the battlefield as a copy of a creature or planeswalker you control, except it enters with an additional +1/+1 counter on it if it's a creature, it enters with an additional loyalty counter on it if it's a planeswalker, and it isn't legendary.
Flash When Tishana's Tidebinder enters the battlefield, counter up to one target activated or triggered ability. If an ability of an artifact, creature, or planeswalker is countered this way, that permanent loses all abilities for as long as Tishana's Tidebinder remains on the battlefield. (Mana abilities can't be targeted.)
When Topography Tracker enters the battlefield, create a Map token. (It's an artifact with "{1}, {T}, Sacrifice this artifact: Target creature you control explores. Activate only as a sorcery.") If a creature you control would explore, instead it explores, then it explores again.
Draw
Svyelun of Sea and Sky has indestructible as long as you control at least two other Merfolk. Whenever Svyelun attacks, draw a card. Other Merfolk you control have ward {1}. (Whenever another Merfolk you control becomes the target of a spell or ability an opponent controls, counter it unless that player pays {1}.)
Spree (Choose one or more additional costs.) + {2} — Put two +1/+1 counters on target creature. + {1} — Target creature gains trample until end of turn. + {1} — Until end of turn, target creature gains "Whenever this creature deals combat damage to a player, draw two cards."
When Zegana, Utopian Speaker enters the battlefield, if you control another creature with a +1/+1 counter on it, draw a card. {4}{G}{U}: Adapt 4. (If this creature has no +1/+1 counters on it, put four +1/+1 counters on it.) Each creature you control with a +1/+1 counter on it has trample.
Evasion
Finisher
Champion a Merfolk (When this enters the battlefield, sacrifice it unless you exile another Merfolk you control. When this leaves the battlefield, that card returns to the battlefield.) Whenever Wanderwine Prophets deals combat damage to a player, you may sacrifice a Merfolk. If you do, take an extra turn after this one.
Land
Hideaway 4 (When this land enters the battlefield, look at the top four cards of your library, exile one face down, then put the rest on the bottom in a random order.) Mosswort Bridge enters the battlefield tapped. {T}: Add {G}. {G}, {T}: You may play the exiled card without paying its mana cost if creatures you control have total power 10 or greater.
Protection
Permanents you control with counters on them gain hexproof and indestructible until end of turn. Prevent all damage that would be dealt to those permanents this turn. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
Proliferate, then choose any number of permanents you control that had a counter put on them this way. Those permanents phase out. (To proliferate, choose any number of permanents and/or players, then give each another counter of each kind already there. Treat phased-out permanents and anything attached to them as though they don't exist until their controller's next turn.)
Spree (Choose one or more additional costs.) + {2} — Mill four cards. You may put up to two creature and/or land cards from among the milled cards into your hand. + {4}{G} — You may put up to two creature cards from your hand onto the battlefield. + {1} — Creatures you control with power 4 or greater gain hexproof and indestructible until end of turn.
Ramp
Removal
Counter target spell. If the spell is countered this way, exile it with three time counters on it instead of putting it into its owner's graveyard. If it doesn't have suspend, it gains suspend. (At the beginning of its owner's upkeep, they remove a time counter. When the last is removed, they may play it without paying its mana cost. If it's a creature, it has haste.)
Kicker {X}. X can't be 0. (You may pay an additional {X} as you cast this spell.) Flying When Thieving Skydiver enters the battlefield, if it was kicked, gain control of target artifact with mana value X or less. If that artifact is an Equipment, attach it to Thieving Skydiver.
Theft
Islandwalk (This creature can't be blocked as long as defending player controls an Island.) Whenever Thada Adel, Acquisitor deals combat damage to a player, search that player's library for an artifact card and exile it. Then that player shuffles. Until end of turn, you may play that card.
Maybeboard
{1}{G}{U}: Put a +1/+1 counter on target creature you control. Whenever one or more +1/+1 counters are put on a creature you control, put that many growth counters on Simic Ascendancy. At the beginning of your upkeep, if Simic Ascendancy has twenty or more growth counters on it, you win the game.