msem-instigator

Whenever one or more nontoken Merfolk you control attack, create a 2/1 red Merfolk creature token.

Scoundreleo enters the battlefield with a +1/+1 counter on it if a creature you controlled dealt combat damage to a player this turn.

{2}{R}: Target creature can't block this turn.

When Sea Hunter enters the battlefield, you may return target creature with mana value 4 or greater to its owner's hand.

Sea Of Skies Beyond enters the battlefield tapped.
Whenever you cast a noncreature spell, put a creation counter on Sea Of Skies Beyond.
{T}, Remove ten creation counters from Sea Of Skies Beyond and sacrifice it: Shuffle your hand into your library, then draw five cards.

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
When Sea's Allure enters the battlefield, gain control of target creature for as long Sea's Allure remains on the battlefield.

Concord — {1}{G}: Until end of turn, Seadrake Rider gets +2/+3 and must be blocked if able. Activate only during your turn, only once each turn, and only if there are four or more card types among permanents you control.

{2}{W}, {T}: Target creature gets +2/+2 and gains first strike until end of turn.

Seafoam Slasher gets +1/+1 and has deathtouch as long seven or more cards are in your graveyard.
Cycling {2} ({2}, Discard this card: Draw a card.)

Flash
{U}, Sacrifice Seal of Approval: Counter target spell unless its controller pays {2}.

First mate—Each nonland card in your starting deck targets or has an ability that targets. (If this card outside the game is your chosen first mate, you may swap it with a card in your opening hand.)
Hexproof
Whenever a permanent you don't control becomes the target of a spell or ability you control, create a token that's a copy of that permanent.

({T}: Add {U}.)
As Seaside Grotto enters the battlefield, you may pay 1 life. If you don't, Seaside Grotto enters the battlefield tapped.
Basic lands you control have “{T}: Add {U}.”

Whenever Seasoned Lookout deals combat damage to a player, it voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)
Whenever Seasoned Lookout phases in, reveal the top card of your library. If it's a land card, put it onto the battlefield. Otherwise, put it into your hand.

Vigilance
Seawise Swain gets +1/+1 as long as you control an artifact.
Seawise Swain gets +1/+1 as long as you control an enchantment.

({T}: Add {G} or {U}.)
Secluded Cascade enters the battlefield tapped unless you control two or more basic lands.

({T}: Add {W} or {U}.)
Whenever Sedate Tundra becomes tapped, you lose 1 life unless you control two or more basic lands.

Whenever a creature enters the battlefield under your control, if it wasn't cast from your hand, you may mill a number of cards equal to its mana value. If you do, you may return a noncreature card milled this way from your graveyard to your hand.

(Transcend as a sorcery to gain that type and a +1/+1 counter.)
Ward {2}
{2}{U}: Demigod — Exchange a card in your hand and a noncreature, nonland card in your graveyard.
{6}{U}: God — Exchange control of two target permanents that share a card type.

Pressure {2}{G}{G} (You may cast this spell for its pressure cost if there are seven or more cards in your graveyard.)
Trample, hexproof
Shambling Dreadreef's power and toughness are each equal to the total number of land cards you own on the battlefield and in your graveyard.

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever you cast a noncreature spell, create an Idol token. (It's an artifact with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)

Concord — {T}: Tap target nonland permanent. Activate only if there are four or more card types among permanents you control.

When Sheltered Diviner enters the battlefield, each player mills three cards.

({T}: Add {G} or {U}.)
Whenever Shifting Glade becomes tapped, you lose 1 life unless you control two or more basic lands.