msem-instigator

Equipped creature gets +1/+0 and has menace.
Equip {2}

{T}, Sacrifice Sinister Thicket: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.
{4}, Exile Sinister Thicket from your graveyard: Draw a card. Activate only as a sorcery.

Counter target spell.
Forebode. (Create a tapped colorless Omen artifact token with “{T}, Discard a card, Sacrifice this artifact: Draw a card.”)

Enchant creature
When Sink Beneath the Sands enters, target player loses 2 life and you gain 2 life.
Enchanted creature gets -4/-4.

Target creature gets -2/-2 until end of turn. Scry 1.

Enchant non-Army creature
When Sinking Pessimism enters, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.
Enchanted creature has “{4}: Untap this creature. If it untapped this way, put a stun counter on it.”

Death knell — You gain 2 life. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Draw a card, then forebode. (Create a tapped colorless Omen artifact token with “{T}, Discard a card, Sacrifice this artifact: Draw a card.”)

Haste
Sixpod Sprinter can't be blocked except by creatures with flying or reach.

Vigilance
When Skaab Grappler enters, you may exile two creature cards from your graveyard. When you do, return another target creature to its owner's hand.

When Skelephant enters, you may mill three cards.
Foresight — Whenever you draw your second card each turn, you may put target creature card from your graveyard on top of your library.

{T}, Sacrifice Skeletal Oasis: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

When Skeletal Shredwing enters, target creature can't block this turn.
Revive {5}{R} ({5}{R}, 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.)

Flying
Whenever Skhorvall Patrol deals combat damage to a player, look at the top card of that player's library.

When Skhorvall Raider enters, if you control an embattled creature, Skhorvall Raider deals 2 damage to any target. (A creature is embattled if it has a counter on it or has a permanent you control attached to it.)

Sacrifice another creature: Put a +1/+1 counter on Skieryg Menace.

Skin from Bone deals damage to target creature equal to your fealty to Mountains. (Each Mountain you control counts towards your fealty to Mountains.)

When Skittering Atrocity enters, up to one target creature gets -X/-X until end of turn, where X is the number of Horrors you control.

Reach

Advance {3}{R}{R} (Cast this for {3}{R}{R} to advance it as it enters. {3}{R}{R}: Advance this as a sorcery.)
Flying, haste
Whenever Skiviar Scraplord advances, put a +1/+1 counter on it.

Until end of turn, target creature you control gets +1/+1 and target creature an opponent controls gets -1/-1.

Menace
When this creature enters, put a -1/-1 counter on a creature you control.

Flying
Death knell — Skullhoarder Dragon gets +2/+0 until end of turn. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Flying
When Sky-Torrent Navigator enters, forebode. If you control two or more Pirates, forebode again. (To forebode, create a tapped colorless Omen artifact token with “{T}, Discard a card, Sacrifice this artifact: Draw a card.”)

Flying
When Skyborne Strategist enters, amass Soldiers 1. (Put a +1/+1 counter on an Army you control. It's also a Soldier. If you don't control one, create an 0/0 black Soldier Army creature token first.)