msem-instigator

Whenever you discard an instant or sorcery card, you may cast it from your graveyard. (You still pay its costs.)
Spells you cast from anywhere other than your hand cost {1} less to cast.

{2}, Return a creature you control to its owner's hand: Smooth Musician gets +1/+1 and gains lifelink until end of turn.

Enchant creature
Enchanted creature has poisonous 3. (Whenever that creature deals combat damage to a player, that player gets three poison counters.)

Flash
Wither (This deals damage to creatures in the form of -1/-1 counters.)

Creatures with power less than Sneaking Peak's power can't block it.
At the beginning of your end step, if Sneaking Peak didn't attack this turn, put a +1/+1 counter on it.

Tap up to two target permanents. They don't untap during their controller's next untap step. You may put Snooze into its owner's library third from the top.

Snow Drift enters tapped. When it does, 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 this, then you may put that card into your graveyard.)
{T}: Add {U}.
{4}{U}: Snow Drift becomes an Elemental creature. (This effect lasts indefinitely.)

Prowess
When Snowline Ascetic enters, you may put target instant or sorcery card from your graveyard on top of your library.

Bleed {0} (You may cast this spell for its bleed cost if an opponent has lost life this turn.)
Look at target player's hand.
Draw a card.

({T}: Add {U} or {B}.)
Sodden Cavern enters tapped.

Target player mills six cards. Return each creature that shares a name with a card milled this way to its owner's hand.
Prophecy {1}{U} (Once each turn, you may exile a card with prophecy as you reveal it. When you do, you may cast it for its prophecy cost as long as it's exiled.)

When Sofia's Swashbucklers enters, you may return target card with foretell from your graveyard to your hand.

Solar Flare deals 3 damage to any target. You gain 3 life.

Destroy target artifact or enchantment. You gain 4 life.

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

As Soli Fortress enters, you may pay 1 life. If you don't, Soli Fortress enters tapped.
Satrapies you control have “{T}: Add {W}.”

(As an Encounter enters, choose an opponent to protect it. You and others can attack it. When defense counters are removed, create a 0/0 Army token if you don't control one, then put that many +1/+1 counters on a Army you control.)
When Soli Rising enters, reveal cards from the top of your library until you reveal a creature card. Put that card into your hand and the rest on the bottom of your library in a random order.

Tap all lands target player controls and that player loses all unspent mana.

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.”)

({T}: Add {B} or {G}.)
Somber Marsh enters tapped.

When Somnambulance enters, each player conjures a card named Fatigue and puts it on top of their library. (It's a colorless sorcery that costs {2} with “Draw a card.”)
Whenever a player casts a spell named Fatigue, you may draw a card. If you do, you lose 1 life.

Enchant permanent
Enchanted permanent is a land with “{T}: Add {G}{W}{U}” and loses all other card types and abilities.
{1}, Discard Song of Shoals: Add one mana of any color. Scry 1.

Create a 4/4 red Dragon creature token with flying.
Showcase {5}{U}{R} (Then if you cast this for its showcase cost, create a 1/1 red Bard creature token that can't block and exile this. Whenever it attacks, you may cast a free copy of this.)

Creatures you control get +2/+2 and gain trample until end of turn.

Put three +1/+1 counters on target creature you control. It voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)