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Commander
Arixmethes, Slumbering Isle enters the battlefield tapped with five slumber counters on it. As long as Arixmethes has a slumber counter on it, it's a land. (It's not a creature.) Whenever you cast a spell, you may remove a slumber counter from Arixmethes. {T}: Add {G}{U}.
Artifact
Card Advantage
Counterspell
Creatures of the Deep
When Biblioplex Kraken enters the battlefield, scry 3. (Look at the top three cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.) Whenever Biblioplex Kraken attacks, you may return another creature you control to its owner's hand. If you do, Biblioplex Kraken can't be blocked this turn.
Deep-Sea Kraken can't be blocked. Suspend 9—{2}{U} (Rather than cast this card from your hand, you may pay {2}{U} and exile it with nine time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost. It has haste.) Whenever an opponent casts a spell, if Deep-Sea Kraken is suspended, remove a time counter from it.
Kicker {1}{U} (You may pay an additional {1}{U} as you cast this spell.) When Slinn Voda, the Rising Deep enters the battlefield, if it was kicked, return all creatures to their owners' hands except for Merfolk, Krakens, Leviathans, Octopuses, and Serpents.
Draw
Enchantment
Enchant creature {U}: Untap enchanted creature. {U}: Enchanted creature gains flying until end of turn. {U}: Enchanted creature gains shroud until end of turn. (It can't be the target of spells or abilities.) {1}: Enchanted creature gets +1/-1 or -1/+1 until end of turn.
Land
Dark Depths enters the battlefield with ten ice counters on it. {3}: Remove an ice counter from Dark Depths. When Dark Depths has no ice counters on it, sacrifice it. If you do, create Marit Lage, a legendary 20/20 black Avatar creature token with flying and indestructible.
If you control two or more other lands, Hall of Storm Giants enters the battlefield tapped. {T}: Add {U}. {5}{U}: Until end of turn, Hall of Storm Giants becomes a 7/7 blue Giant creature with ward {3}. It's still a land. (Whenever it becomes the target of a spell or ability an opponent controls, counter it unless that player pays {3}.)
Littjara Mirrorlake enters the battlefield tapped. {T}: Add {U}. {2}{G}{G}{U}, {T}, Sacrifice Littjara Mirrorlake: Create a token that's a copy of target creature you control, except it enters the battlefield with an additional +1/+1 counter on it. Activate only as a sorcery.
Path of Ancestry enters the battlefield 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.)
Planeswalker
+1: Untap up to one target creature and up to one target land. −2: Reveal the top four cards of your library. You may put a creature card and/or a land card from among them into your hand. Put the rest into your graveyard. −8: You get an emblem with "Whenever a creature enters the battlefield under your control, you may have it fight target creature." Then create three 8/8 blue Octopus creature tokens.
+1: Until your next turn, prevent all damage that would be dealt to and dealt by target permanent an opponent controls. −1: Draw a card. You may play an additional land this turn. −5: You get an emblem with "At the beginning of your end step, create a 9/9 blue Kraken creature token."
Ramp
Search your library for a basic land card, put it onto the battlefield, then shuffle. Suspend 2—{G} (Rather than cast this card from your hand, you may pay {G} and exile it with two time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost.)
Ramp-ish
Reduction Costs
Regenerate
Removals
Tribal Focus
If you would scry a number of cards, scry that many cards plus one instead. {3}{G/U}: Look at the top card of your library. If it's a Kraken, Leviathan, Octopus, or Serpent creature card, you may put it onto the battlefield. If you don't put the card onto the battlefield, you may put it on the bottom of your library.
Vigilance, ward {3} Whenever you cast a Kraken, Leviathan, Octopus, or Serpent spell from your hand, look at the top X cards of your library, where X is that spell's mana value. You may cast a spell with mana value less than X from among them without paying its mana cost. Put the rest on the bottom of your library in a random order.
At the beginning of your upkeep, you may look at the top card of your library. If it's a creature card, you may reveal it and put a quest counter on Quest for Ula's Temple. At the beginning of each end step, if there are three or more quest counters on Quest for Ula's Temple, you may put a Kraken, Leviathan, Octopus, or Serpent creature card from your hand onto the battlefield.
Sideboard
When Garruk's Uprising enters the battlefield, if you control a creature with power 4 or greater, draw a card. Creatures you control have trample. (They can deal excess combat damage to the player or planeswalker they're attacking.) Whenever a creature with power 4 or greater enters the battlefield under your control, draw a card.
Cumulative upkeep {1} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.) Whenever an opponent casts a noncreature spell, you may draw a card unless that player pays {4}.
Maybeboard
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.)
At the beginning of combat on your turn, up to one target land you control becomes an X/X green and blue Bird creature with flying and haste until end of turn, where X is the number of cards in your hand. It's still a land. Whenever a land creature you control deals combat damage to a player, draw a card.
Choose one. If you control a commander as you cast this spell, you may choose both. • Target opponent chooses a creature they control. You gain control of it. • Choose a creature you control. Each other creature you control becomes a copy of that creature until end of turn.
The Watcher in the Water enters the battlefield tapped with nine stun counters on it. (If a permanent with a stun counter would become untapped, remove one from it instead.) Whenever you draw a card during an opponent's turn, create a 1/1 blue Tentacle creature token. Whenever a Tentacle you control dies, untap up to one target Kraken and put a stun counter on up to one target nonland permanent.