msem-instigator

If you control seven or more creatures, you may cast Sociable Giant without paying its mana cost.
Trample

Whenever you cast a noncreature spell, this creature gets +1/+0 and gains first strike until end of turn. Surveil 1.

Flying

When Solar Wisp enters, put a +1/+1 counter on target creature.

Whenever Solemn Stag perseveres, put two +1/+1 counters on it. (A creature perseveres when it survives damage.)

When Soli Chronicler enters, judge another target creature you control. (Honor it by putting a +1/+1 counter on it or condemn it by sacrificing it to draw a card.)

At the beginning of each player's upkeep, that player may pay {2} or 2 life. If they don't, they return a permanent they control to its owner's hand.

Whenever Solvent Tide attacks, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)

Whenever another creature you control enters, target opponent loses 1 life and you gain 1 life.
Sacrifice Somber River Bearer: Creatures you control gain indestructible until end of turn.

Somberwald Rider can't be blocked by creatures with power 2 or less.
{2}{G}: Transform Somberwald Rider. Activate only if it's attacking.

Alpha (You may have this creature enter with a +1/+1 counter on it. If you do, it becomes alpha and can only attack alone.)
Whenever a creature you control attacks alone, untap all lands you control.

Reach
{3}, Exile Sorrowful Seeker from your hand: Draw a card. You may cast this card for as long as it remains exiled.

Flying, lifelink
Morph {4}{B}
When Soul Cajoler is turned face up, manifest the bottom card of your library. (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)

Whenever another creature enters, you gain 1 life.

Flying, trample, haste
{W}{U}{B}{R}{G}: Create a token that's a copy of another target creature except it's all colors and has haste.
{W}{U}{B}{R}{G}, Exile Soul of Iscathia from your graveyard: Create a token that's a copy of target creature except it's all colors and has haste.

Protection from colored spells
{5}{C}{C}: Exile target colored permanent.
{5}{C}{C}, Exile Soul of Muetar from your graveyard: Exile target colored permanent.

Whenever another creature enters, you may gain 1 life.

Advance {2}{R} (Cast this for {2}{R} to advance it as it enters. {2}{R}: Advance this as a sorcery.)
Haste
Whenever Soulblade Warrior advances, it gets +2/+0 until end of turn.

{1}, Sacrifice a creature: Each opponent loses 1 life and you gain 1 life.

Soulsmoke Adept gets +1/+0 and has lifelink as long as it's embattled. (A creature is embattled if it has a counter on it or has a permanent you control attached to it.)

Whenever Sower of Sabne's Harvest enters or attacks, align X, where X is your fealty to Forests. (Choose modes with total ★ less than or equal to the number of Forests you control. You may choose modes multiple times.)
• ★★ — Put a +1/+1 counter on target creature.
• ★★★ — Exile target artifact or enchantment.
• ★★★★★ — You gain 5 life.

Whenever Sower of Virulent Thoughts attacks, you may return a creature you control to its owner's hand. If you do, put a +1/+1 counter on target creature you control.

Chosen one ({W}{U}{B}{R}{G}: Transform this permanent.)
When Sower of Wishes enters, reveal the top three cards of your library. Put all land cards revealed this way into your hand and the rest back on top in any order.

{2}: Sparkline Dancer gains flying until end of turn.

Sparkmage Prodigy has first strike as long as it's your turn.
Whenever you cast a cryptic spell, Sparkmage Prodigy deals 1 damage to each opponent. (Artifacts, enchantments, and modal spells are cryptic.)