msem-instigator

Whenever you cast a spell with mana value greater than Script Jockey's power, put a +1/+1 counter on Script Jockey.

Scrollsnatcher can't block and can't be blocked.
Whenever Scrollsnatcher deals combat damage to a player, look at the top card of each library, then put any number of those cards into their owner's graveyards.

Haste
Frenzy 2 (Whenever this creature attacks and isn't blocked, it gets +2/+0 until end of turn.)

Scrounging Dunes enters tapped.
{T}: Add {W} or {B}.
{1}{W}{B}: Until end of turn, Scrounging Dunes becomes a Wurm creature with lifelink in addition to its other types.
Scavenge {1}{W}{B}

At the beginning of combat on your turn, if you cast both a creature spell and a noncreature spell this turn, untap up to one target land you control. It becomes a 3/1 red Sand creature with haste in addition to its other types until end of turn.

Flash
When Scuripi Lookout enters, target creature gains hexproof until end of turn.
Whenever one or more creatures you control attack, you may return Scuripi Lookout to its owner's hand.

Vigilance
{T}: Add {G}.

Whenever you sacrifice a permanent, you may pay {1}. If you do, you draw a card and you gain 1 life.

At the beginning of your upkeep, if six or more cards are in your graveyard, you may transform Scuttling Gnat.

Play with the top card of your library revealed.
You may cast Elemental spells from the top of your library.

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

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.)

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.

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.)

Haste
Shimmer (Whenever this creature attacks, create a tapped and attacking token that's a copy of it. Exile the token at the beginning of the end step.)

When Second-Stage Actress enters, audition for white. (Create a 1/1 Construct Actor creature token, a Scout Role giving 2/3 and vigilance, or a Cleric Role giving 2/2 and lifelink. Roles replace older ones.)
As long as it's your turn, Second-Stage Actress has first strike.

Flying
Instant and sorcery spells you cast cost {1} less to cast.
{2}{U}: Target player mills two cards.

Glorious end — When Seeker of Conto Sika dies, if it was tapped, draw a card.

When Seeker of Far-Off Lands enters, search target opponent's library for a land card, put it onto the battlefield tapped under your control, then that player shuffles.

Flying
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever Seeker of Ruins deals combat damage to a player, put a +1/+1 counter on it.

As long as you control four or more lands, Seeker of Untouched Beauty gets +2/+2 and has trample.
When Seeker of Untouched Beauty dies, you may search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

When Seishin Rageshaper enters, create a 2/2 colorless Spirit creature token.
Colorless creatures you control have haste.

When Seishin Smokeshaper enters, create a 2/2 colorless Spirit creature token.
Whenever you cast a noncreature spell, colorless creatures you control get +1/+1 until end of turn.

When Seishin Tidekeeper enters, create a 2/2 colorless Spirit creature token.
Colorless creatures you control have flying.