Serum Snare

Return target nonland permanent to its owner's hand. If that permanent had mana value 3 or less, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Sentinel's Eyes

Enchant creature
Enchanted creature gets +1/+1 and has vigilance.
Escape—{W}, Exile two other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Crystal Grotto

When Crystal Grotto enters the battlefield, scry 1.
{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Sky Diamond

Sky Diamond enters the battlefield tapped.
{T}: Add {U}.

Fluros of Myra's Marvels

As Fluros of Myra's Marvels enters the battlefield, choose a number greater than 0.
Whenever you cast a spell with power, toughness, or loyalty equal to the chosen number, target artifact or creature becomes an artifact creature with base power and toughness X/X until end of turn, where X is the chosen number.
Partner

Azorius Chancery

Azorius Chancery enters the battlefield tapped.
When Azorius Chancery enters the battlefield, return a land you control to its owner's hand.
{T}: Add {W}{U}.

Serum Sovereign

Flying
Whenever you cast a noncreature spell, put an oil counter on Serum Sovereign.
{U}, Remove an oil counter from Serum Sovereign: Draw a card, then scry 2.

Donal, Herald of Wings

Whenever you cast a nonlegendary creature spell with flying, you may copy it, except the copy is a 1/1 Spirit in addition to its other types. Do this only once each turn. (The copy becomes a token.)

Sea Gate, Reborn

As Sea Gate, Reborn enters the battlefield, you may pay 3 life. If you don't, it enters the battlefield tapped.
{T}: Add {U}.

Sea Gate Restoration

Draw cards equal to the number of cards in your hand plus one. You have no maximum hand size for the rest of the game.

Sea Gate Restoration // Sea Gate, Reborn

Stenn, Paranoid Partisan

As Stenn, Paranoid Partisan enters the battlefield, choose a card type other than creature or land.
Spells you cast of the chosen type cost {1} less to cast.
{1}{W}{U}: Exile Stenn. Return it to the battlefield under its owner's control at the beginning of the next end step.

Supreme Verdict

This spell can't be countered.
Destroy all creatures.

Scour All Possibilities

Scry 2, then draw a card.
Flashback {4}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Command Tower

{T}: Add one mana of any color in your commander's color identity.

Siren Stormtamer

Flying
{U}, Sacrifice Siren Stormtamer: Counter target spell or ability that targets you or a creature you control.

Elminster

Whenever you scry, the next instant or sorcery spell you cast this turn costs {X} less to cast, where X is the number of cards looked at while scrying this way.
+2: Draw a card, then scry 2.
−3: Exile the top card of your library. Create a number of 1/1 blue Faerie Dragon creature tokens with flying equal to that card's mana value.
Elminster can be your commander.

Solidarity

Creatures you control get +0/+5 until end of turn.

Island

Spark Double

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.

Terramorphic Expanse

{T}, Sacrifice Terramorphic Expanse: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Solve the Equation

Search your library for an instant or sorcery card, reveal it, put it into your hand, then shuffle.

Temple of Enlightenment

Temple of Enlightenment enters the battlefield tapped.
When Temple of Enlightenment enters the battlefield, scry 1.
{T}: Add {W} or {U}.

Sphinx's Revelation

Sentinel

{0}: Change Sentinel's base toughness to 1 plus the power of target creature blocking or blocked by Sentinel. (This effect lasts indefinitely.)

Swords to Plowshares

Exile target creature. Its controller gains life equal to its power.

Slaughter the Strong

Each player chooses any number of creatures they control with total power 4 or less, then sacrifices all other creatures they control.

Sentinel's Mark

Flash
Enchant creature
Enchanted creature gets +1/+2 and has vigilance.
Addendum — When Sentinel's Mark enters the battlefield, if you cast it during your main phase, enchanted creature gains lifelink until end of turn.

Secret Rendezvous

You and target opponent each draw three cards.

Siren's Ruse

Exile target creature you control, then return that card to the battlefield under its owner's control. If a Pirate was exiled this way, draw a card.

Sprite Noble

Flying
Other creatures you control with flying get +0/+1.
{T}: Other creatures you control with flying get +1/+0 until end of turn.

Staggering Insight

Enchant creature
Enchanted creature gets +1/+1 and has lifelink and "Whenever this creature deals combat damage to a player, draw a card."

Skeleton Key

Equipped creature has skulk. (It can't be blocked by creatures with greater power.)
Whenever equipped creature deals combat damage to a player, you may draw a card. If you do, discard a card.
Equip {2}

Psionic Pulse

Counter target noncreature spell. (Then exile this card. You may cast the creature later from exile.)

Sapphire Dragon

Flying
Whenever Sapphire Dragon attacks or blocks, scry 2.

Sapphire Dragon // Psionic Pulse

Sphinx of Enlightenment

Flying
When Sphinx of Enlightenment enters the battlefield, target opponent draws a card and you draw three cards.

Myriad Landscape

Myriad Landscape enters the battlefield tapped.
{T}: Add {C}.
{2}, {T}, Sacrifice Myriad Landscape: Search your library for up to two basic land cards that share a land type, put them onto the battlefield tapped, then shuffle.

Prairie Stream

({T}: Add {W} or {U}.)
Prairie Stream enters the battlefield tapped unless you control two or more basic lands.

Siren of the Fanged Coast

Flying
Tribute 3 (As this creature enters the battlefield, an opponent of your choice may put three +1/+1 counters on it.)
When Siren of the Fanged Coast enters the battlefield, if tribute wasn't paid, gain control of target creature.

Slagwurm Armor

Equipped creature gets +0/+6.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery. This card enters the battlefield unattached and stays on the battlefield if the creature leaves.)

Sol Ring

Skyscanner

Flying
When Skyscanner enters the battlefield, draw a card.

Shark Typhoon

Whenever you cast a noncreature spell, create an X/X blue Shark creature token with flying, where X is that spell's mana value.
Cycling {X}{1}{U} ({X}{1}{U}, Discard this card: Draw a card.)
When you cycle Shark Typhoon, create an X/X blue Shark creature token with flying.

Plains

Swiftfoot Boots

Equipped creature has hexproof and haste. (It can't be the target of spells or abilities your opponents control.)
Equip {1}

Rasaad yn Bashir

Each creature you control assigns combat damage equal to its toughness rather than its power.
Whenever Rasaad yn Bashir attacks, if you have the initiative, double the toughness of each creature you control until end of turn.
Choose a Background (You can have a Background as a second commander.)

Sigiled Starfish

{T}: Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Solemn Simulacrum

When Solemn Simulacrum enters the battlefield, you may search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.
When Solemn Simulacrum dies, you may draw a card.

Siren Reaver

Raid — This spell costs {1} less to cast if you attacked this turn.
Flying

Shield-Wall Sentinel

Defender
When Shield-Wall Sentinel enters the battlefield, you may search your library for a creature card with defender, reveal it, put it into your hand, then shuffle.

Shrieking Drake

Flying
When Shrieking Drake enters the battlefield, return a creature you control to its owner's hand.

Sphinx of Foresight

You may reveal this card from your opening hand. If you do, scry 3 at the beginning of your first upkeep.
Flying
At the beginning of your upkeep, scry 1.

Strength-Testing Hammer

Whenever equipped creature attacks, roll a six-sided die. That creature gets +X/+0 until end of turn, where X is the result. Then if it has the greatest power or is tied for greatest power among creatures on the battlefield, draw a card.
Equip {3}

Sinister Sabotage

Counter target spell.
Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)

Spectrum Sentinel

Protection from multicolored (This creature can't be blocked, targeted, dealt damage, enchanted, or equipped by anything multicolored.)
Whenever a nonbasic land enters the battlefield under an opponent's control, you gain 1 life.

Storm Herd

Create X 1/1 white Pegasus creature tokens with flying, where X is your life total.

Evolving Wilds

{T}, Sacrifice Evolving Wilds: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Sungold Sentinel

Whenever Sungold Sentinel enters the battlefield or attacks, exile up to one target card from a graveyard.
Coven — {1}{W}: Choose a color. Sungold Sentinel gains hexproof from that color until end of turn and can't be blocked by creatures of that color this turn. Activate only if you control three or more creatures with different powers.

Scars of the Veteran

You may exile a white card from your hand rather than pay this spell's mana cost.
Prevent the next 7 damage that would be dealt to any target this turn. If it's a creature, put a +0/+1 counter on it for each 1 damage prevented this way at the beginning of the next end step.

Stoneskin

Flash
Enchant creature
Enchanted creature gets +0/+10.

Swan Song

Counter target enchantment, instant, or sorcery spell. Its controller creates a 2/2 blue Bird creature token with flying.

Sephara, Sky's Blade

You may pay {W} and tap four untapped creatures you control with flying rather than pay this spell's mana cost.
Flying, lifelink
Other creatures you control with flying have indestructible. (Damage and effects that say "destroy" don't destroy them.)

Swarm Intelligence

Whenever you cast an instant or sorcery spell, you may copy that spell. You may choose new targets for the copy.

Spectral Sailor

Flash (You may cast this spell any time you could cast an instant.)
Flying
{3}{U}: Draw a card.

Spidersilk Net

Equipped creature gets +0/+2 and has reach. (It can block creatures with flying.)
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Silundi Isle

Silundi Isle enters the battlefield tapped.
{T}: Add {U}.

Silundi Vision

Look at the top six cards of your library. You may reveal an instant or sorcery card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Silundi Vision // Silundi Isle

Solar Tide

Choose one —
• Destroy all creatures with power 2 or less.
• Destroy all creatures with power 3 or greater.
Entwine—Sacrifice two lands. (Choose both if you pay the entwine cost.)

Soul Snare

{W}, Sacrifice Soul Snare: Exile target creature that's attacking you or a planeswalker you control.

Silent Sentinel

Flying
Whenever Silent Sentinel attacks, you may return target enchantment card from your graveyard to the battlefield.

Serum Visions

Shield Mate

Sacrifice Shield Mate: Target creature gets +0/+4 until end of turn.

Katerina of Myra's Marvels

As Katerina of Myra's Marvels enters the battlefield, choose a letter.
Whenever you cast a spell whose name begins with the chosen letter, create a white 2/2 Cat creature token with flying.
Partner (You can have two commanders if they both have partner.)

Sun Titan

Vigilance
Whenever Sun Titan enters the battlefield or attacks, you may return target permanent card with mana value 3 or less from your graveyard to the battlefield.

Skullclamp

Equipped creature gets +1/-1.
Whenever equipped creature dies, draw two cards.
Equip {1}

Starlight Spectacular

Parade! — At the beginning of combat on your turn, choose creatures you control one at a time until each creature you control has been chosen. Each of those creatures gets +1/+1 until end of turn for each creature chosen before it. (Places everyone! The first creature in line gets +0/+0.)

Port Town

As Port Town enters the battlefield, you may reveal a Plains or Island card from your hand. If you don't, Port Town enters the battlefield tapped.
{T}: Add {W} or {U}.

Shapeshifter

As Shapeshifter enters the battlefield, choose a number between 0 and 7.
At the beginning of your upkeep, you may choose a number between 0 and 7.
Shapeshifter's power is equal to the last chosen number and its toughness is equal to 7 minus that number.