msem-instigator

Shock Blast deals X damage to target creature, where X is the number of instant and sorcery cards in your graveyard.

Reveal the top four cards of your library and separate them into two piles. An opponent chooses one of those piles. Put that pile into your hand. Shuffle the other and up to four cards from your graveyard into your library.

Vanguard — Whenever a creature you control attacks alone, Shocking Ewe deals 2 damage to defending player.

When Shokan Captain enters, you may search your library for an Equipment card, reveal it, shuffle, then put that card on top.
Art of war — At the beginning of your end step, if Shokan Captain dealt damage to an opponent this turn, put a +1/+1 counter on it.

{3}{U}{U}: Monstrosity 4. (If this creature isn't monstrous, put four +1/+1 counters on it and it becomes monstrous.)
When Shorecrash Octopus becomes monstrous, return target instant or sorcery card from your graveyard to your hand.

Heretic (Whenever you cast a spell that doesn't share a color with this creature, it gets +1/+1 until end of turn.)
When Shoring Naiad enters, if {U} was spent to cast it, you may return an instant or sorcery card from your graveyard to your hand.

Search your library for two basic land cards, reveal them, put them into your hand, then shuffle. (Then exile this card. You may cast the creature later from exile.)

{2/W}{2/W}: Shrapnel Golem deals 3 damage to target creature blocking it.

{T}, Sacrifice an artifact: Add {R}{R}{R}.

Whenever equipped creature deals damage to a creature, that creature gets -2/-2 until end of turn.
Equip {2}

Shrewd Sikoyo can't be blocked by creatures with power 4 or greater.
Shrewd Sikoyo gets +1/+0 as long as you have at least two more cards in hand than any opponent.

Flying, vigilance

Lifelink
Whenever an Aura becomes attached to Shrieking Gnoll, put a +1/+1 counter on Shrieking Gnoll, then target creature an opponent controls gets -1/-1 until end of turn for each +1/+1 counter on Shrieking Gnoll.

Haste
Whenever you sacrifice another permanent, put a +1/+1 counter on Shrine Razer.

{T}: Add {C}.
{T}: Add one mana of any color. Activate only if you've cast a noncreature spell this turn.

Colorless creatures you control get +1/+1.

Return target nonland permanent to its owner's hand.
Inscribed creature gets +1/+1 and has flying.
Inscribe {3}{U} ({3}{U}: Exile this card from your hand inscribed on a creature you control. Whenever that creature attacks, its controller may cast a copy of the inscribed card without paying its mana cost. Inscribe only as a sorcery.)

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)
Shrouded Cackler can't block.

{T}, Pay 1 life: Add one mana of any color. Spend this mana only to cast an instant or sorcery spell.

This spell costs {2} less to cast for each face-down creature you control.
Flying

Shu Qin enters with your choice of a flying or +1/+1 counter on him.
At the beginning of your upkeep, choose a creature in Shu Qin's pack. He gains that creature's abilities until end of turn. (Two or more creatures you control with equal power are in a pack.)

Vigilance
Whenever Sibling of All Elements attacks, it gains flying until end of turn if you control a creature with flying. The same is true for first strike, double strike, deathtouch, indestructible, lifelink, menace, and trample.

When Sift Through False Leads enters, exile the top five cards of your library, then investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)
{2}, Sacrifice a Clue: Put a card exiled with Sift Through False Leads into your hand.

When Sights Beyond enters, scry 3.
Concord — At the beginning of your upkeep, if there are four or more card types among permanents you control, draw a card.

Prowess
{T}: Catalyze, then catalyze again. Activate only if Sigil Slinger's power is 5 or greater. (The next spell you cast this turn costs {2} less to cast.)