msem-instigator

{U}: Sea Strider gets +1/-1 until end of turn.
{U}: Sea Strider gets -1/+1 until end of turn.

Defender
{1}{U}: Sea Wall gets +0/+1 until end of turn.
Whenever Sea Wall perseveres, you may return target creature an opponent controls to its owner's hand. (A creature perseveres when it survives damage.)

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, gain control of target creature for as long Sea's Allure remains on the battlefield.

Flash
When Seabreen Nullmage enters, counter target activated or triggered ability. (A triggered ability uses the words “when,” “whenever,” or “at”. Mana abilities can't be targeted.)

When Seafloor Cache enters and when you sacrifice it, create a Treasure token.
{1}{R}, Sacrifice Seafloor Cache: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

When Seafloor Casket enters and when you sacrifice it, target player mills two cards.
{1}{U}, Sacrifice Seafloor Casket: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

When Seafloor Shrine enters and when you sacrifice it, create a 1/1 white Merfolk creature token.
{1}{W}, Sacrifice Seafloor Shrine: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

When Seafloor Tomb enters and when you sacrifice it, exile up to two target cards from graveyards.
{1}{B}, Sacrifice Seafloor Tomb: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

When Seafloor Wreckage enters and when you sacrifice it, search your library for a basic land card, reveal it, put it into your hand, then shuffle.
{1}{G}, Sacrifice Seafloor Wreckage: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

{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.)

Whenever one or more cards leave your graveyard, put a +1/+1 counter on Seagraf Shamblers.

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

{U}, Sacrifice Seal of Insights: Look at the top four cards of your library. Put one into your hand and the rest on the bottom of your library in any order.

Golden age (If you cast an instant or sorcery spell, activated a nonmana ability, and attacked this turn, enter a renaissance for this game.)
Target creature or planeswalker an opponent controls phases out. If you're in a renaissance, exile it instead.

When Sealed Evil in a Cage enters, exile target creature an opponent controls with mana value 3 or greater until Sealed Evil in a Cage leaves the battlefield.

Sear Flesh deals 3 damage to target creature or planeswalker. If you've cast another spell this turn, it deals 5 damage instead.

Another target player reveals their hand. You may choose a nonland card from it. That player discards that card.
Fuse (You may cast one or both halves of this card from your hand.)

Choose one —
• Destroy target artifact or enchantment.
• Target creature you control explores twice. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then you may put that card in your graveyard.)

Reveal the top five cards of your library. You may put an instant or sorcery card from among them into your hand. Put the rest into your graveyard.

Search your library for an enchantment card, reveal it, put it into your hand, then shuffle.

At the beginning of your upkeep, surveil 1. (Look at the top card of your library. You may put that card into your graveyard.)
{2}{U}{U}, Sacrifice Search for a New Land: Draw two cards.

{T}: Add one mana of any color.
{2}, {T}: Draw a card. Activate only if you sacrificed a permanent or discarded a card this turn.

Searing Blasphemy deals 2 damage to target creature. If that creature is an Angel, Searing Blasphemy deals 5 damage instead.