Sigil of Valor

Whenever equipped creature attacks alone, it gets +1/+1 until end of turn for each other creature you control.
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

Skyblade of the Legion

Sandcrafter Mage

When Sandcrafter Mage enters, bolster 1. (Choose a creature with the least toughness among creatures you control and put a +1/+1 counter on it.)

Katerina of Myra's Marvels

As Katerina of Myra's Marvels enters, 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.)

Saber Ants

Whenever Saber Ants is dealt damage, you may create that many 1/1 green Insect creature tokens.

Sigil of the Nayan Gods

Enchant creature
Enchanted creature gets +1/+1 for each creature you control.
Cycling {G/W} ({G/W}, Discard this card: Draw a card.)

Sandwurm Convergence

Creatures with flying can't attack you or planeswalkers you control.
At the beginning of your end step, create a 5/5 green Wurm creature token.

Sinstriker's Will

Enchant creature
Enchanted creature has "{T}: This creature deals damage equal to its power to target attacking or blocking creature."

Hardy of Myra's Marvels

As Hardy of Myra's Marvels enters, choose a number.
Whenever you cast a spell with the chosen number of lines of flavor text, put that many +1/+1 counters on target creature.
Partner (You can have two commanders if both have partner.)

Silent Sentinel

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

Siege Veteran

At the beginning of combat on your turn, put a +1/+1 counter on target creature you control.
Whenever another nontoken Soldier you control dies, create a 1/1 colorless Soldier artifact creature token.

Saproling Migration

Kicker {4} (You may pay an additional {4} as you cast this spell.)
Create two 1/1 green Saproling creature tokens. If this spell was kicked, create four of those tokens instead.

Saltblast

Safe Passage

Prevent all damage that would be dealt to you and creatures you control this turn.

Safehold Elite

Persist (When this creature dies, if it had no -1/-1 counters on it, return it to the battlefield under its owner's control with a -1/-1 counter on it.)

Single Combat

Each player chooses a creature or planeswalker they control, then sacrifices the rest. Players can't cast creature or planeswalker spells until the end of your next turn.

Sanctified Charge

Creatures you control get +2/+1 until end of turn. White creatures you control also gain first strike until end of turn. (They deal combat damage before creatures without first strike.)

Saddled Rimestag

Saddled Rimestag gets +2/+2 as long as you had another creature enter the battlefield under your control this turn.

Salvage

Put target card from your graveyard on top of your library.

Silent Gravestone

Cards in graveyards can't be the targets of spells or abilities.
{4}, {T}: Exile Silent Gravestone and all cards from all graveyards. Draw a card.

Silverback Shaman

Trample
When Silverback Shaman dies, draw a card.

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.

Sky Crier

Flying, lifelink
{3}{W}: You and target opponent each draw a card.

Signal Pest

Battle cry (Whenever this creature attacks, each other attacking creature gets +1/+0 until end of turn.)
Signal Pest can't be blocked except by creatures with flying or reach.