msem-instigator

Haste
Whenever a creature attacks, Slashing Pangolin gets +1/+1 until end of turn.

Improve {1}{B/R} ({1}{B/R}: Put a +1/+1 counter on this creature. This costs {1} more to activate for each +1/+1 counter on it. Improve only as a sorcery.)
At the beginning of each player's combat, you may have that player create two goaded 1/1 red Goblin creature tokens with haste. Exile those tokens at end of combat. (Until your next turn, they attack each combat if able and attack a player other than you if able.)

Slaughterkeg can't attack or block.
While an opponent is choosing targets as part of casting a spell they control or activating an ability they control, that player must choose at least Slaughterkeg if able.
When Slaughterkeg dies, draw a card.

This spell costs {1}{B} less to cast if it targets a creature with indestructible.
Exile target creature.

Enchant land
Whenever enchanted land is tapped for mana, its controller adds {G}.
{G}, Sacrifice Sleeping Dryads: Draw a card.

Enchant opponent
Whenever you attack enchanted player, look at the top X cards of your library, where X is one plus the number of attacking creatures you control. Put one of those cards into your hand and the rest on the bottom of your library in any order.

You may change the targets of target instant spell, sorcery spell, or activated ability.

Until end of turn, target player can't cast noncreature spells, and that player can't activate abilities that aren't mana abilities.
Draw a card.

Flash
When Slippery Assassin enters, counter target spell if its mana value is less than or equal to Slippery Assassin's power.
Sacrifice another creature: Put a +1/+1 counter on Slippery Assassin.

Ward {2}
Slippery Bunyip can't be blocked except by creatures with Adventures.

Slipsand Oasis enters tapped unless you played it from exile.
{T}: Add {W} or {B}.

Whenever a creature enters without counters during your next turn, you may put two +1/+1 counters on it. (Then exile this card. You may cast the creature later from exile.)

{2}, Remove a +1/+1 counter from a creature you control: Draw a card.

Advance {5}{B}{B} (Cast this for {5}{B}{B} to advance it as it enters. {5}{B}{B}: Advance this as a sorcery.)
Whenever Sluiceway Collector advances, put a permanent card from a graveyard onto the battlefield under your control.

Defender
Whenever Slumbering Bear is dealt damage, put a +1/+1 counter on it.
Remove a +1/+1 counter from Slumbering Bear: It can attack as though it didn't have defender this turn.

You draw a card and you lose 1 life. Exile Small Luxuries.
At the beginning of your upkeep, you may return Small Luxuries from your graveyard to your hand.

When Smashing Brute enters, target nonbasic permanent loses all abilities until your next turn.
Foresight — Whenever you draw your second card each turn, you may exile Smashing Brute. If you do, return it to the battlefield.

Exile target permanent. Smite the Unruly deals damage equal to that permanent's mana value to its controller and you gain that much life.

When Snagged Truck enters, investigate three times. (Create three colorless Clue artifact tokens with “{2}, Sacrifice this: Draw a card.”)
Snagged Truck can't attack or block as long as you control a Clue.
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.)

When Snarling Devil enters, create four 1/1 black Possum Dog creature tokens.
At the beginning of each upkeep, put your choice of a +1/+1, lifelink, menace, trample, or vigilance counter on a token creature you control.

Sneaking Capuchin can't be blocked except by creatures with defender.
{U}: Counter target spell or ability that targets Sneaking Capuchin.

Target opponent discards two cards. You gain 2 life. (Then exile this card. You may cast the creature later from exile.)

Enchant artifact or creature
Enchanted permanent doesn't untap during its controller's untap step.
Whenever you make first contact, scry 1. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)

You may have Snowfield Doppelganger enter as a copy of any creature on the battlefield, except it has “While a player is choosing targets as part of casting a spell, that player must choose at least one other creature that shares a name with this creature if able.”