msem-instigator

Enchant creature or land
Whenever enchanted permanent becomes tapped for the first time each turn, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then put the card back or put it into your graveyard.)

Scholar of the Staff can't block.
Inspired — Whenever Scholar of the Staff becomes untapped, up to one target creature can't block this turn.

Attacking creatures get -3/-0 until end of turn.

Schoolmaster gets +1/+1 for each other Merfolk you control.
When Schoolmaster enters, create a 1/1 white Merfolk creature token.

Whenever one or more nontoken Merfolk you control attack, create a 2/1 red Merfolk creature token.

When Scion of Fecundity enters, search your library for a creature card with mana value 2 or less, reveal that card, put it into your hand, then shuffle.
Revive—{1}{G}{W}, Discard a card. ({1}{G}{W}, Discard a card, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Scion of the Wild's power and toughness are each equal to the number of creatures you control.

This spell costs {2} less to cast if its target has the same name as a card in a graveyard.
Scoff deals 3 damage to any target.

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Scorchland Vandal attacks each combat if able.

Wither (This deals damage to creatures in the form of -1/-1 counters.)
Whenever a creature with wither deals combat damage to an opponent, Scorchmark Seer deals 2 damage to that player.

When Scorpid Sniper enters, you may have target creature get -1/-1 until end of turn.

(Add a quest counter as this enters. Sacrifice after 3.)
Whenever you cast a noncreature spell, add a quest counter.
Draw a card, then discard a card.
Return up to two target instant and/or sorcery cards from your graveyard to your hand.
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Engorge (As this enters, mill three cards, then exile a card from among the milled cards.)
When Scourge of the Deadlands enters, if it engorged a creature card, each other player sacrifices a creature.

Reveal the bottom four cards of your library. Choose up to one permanent card with mana value 3 or less from among them and put it onto the battlefield. Put the rest on the bottom of your library in any order.

Target opponent reveals their hand. You choose a nonland card from it and exile that card.
Recruit. (Create a 0/0 white Soldier Army creature token, then put a +1/+1 counter on each Army you control.)

Menace
Sacrifice another artifact: Scrap Buggy becomes an artifact creature until end of turn.
Crew 2 (Tap any number of creatures you control with total power 2 or greater: This Vehicle becomes an artifact creature until end of turn.)

Scrap, the Second Myr has all activated abilities of other artifact creatures you control.

When Scrapgear Contraption dies, you may put a +1/+1 counter on target creature.

Sacrifice Scrapyard Raiders: Target artifact gains indestructible until end of turn.

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

Whenever you cast an instant or sorcery spell, put a +1/+1 counter on Scribe of the Sky Scrolls. If you spent four or more mana to cast that spell, Scribe of the Sky Scrolls gains flying until end of turn.

Flying
Whenever you cast a creature spell, if Script Spirit is in your graveyard, you may have it become a copy of that spell until end of turn.

Create two 1/1 white Human creature tokens.
Put a vigilance counter on target creature you control. It becomes a legendary God in addition to its other types.
Exile target creature an opponent controls that's a God or that has power 4 or greater.

When Scrivener of Sha'Iv enters, draw a card and adjudicate. (Create a white Aura enchantment token with enchant creature and “Sacrifice this token: Untap enchanted creature” attached to target creature.)

When Scuripi Collector enters, return target artifact, enchantment, or land card from your graveyard to your hand.