msem-instigator

Steel Hunter can't attack unless you control three or more other artifacts.

Flying

Renown 1 (When this creature deals combat damage to a player, if it isn't renowned, put a +1/+1 counter on it and it becomes renowned.)

When Steelcharm Obliterator enters, choose one —
• Put two +1/+1 counters on Steelcharm Obliterator.
• Target creature you control fights another target creature.

Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.)
Whenever Steelhoof Battlechief attacks, another target attacking creature you control gets +2/+2 and gains trample until end of turn.

Trial 3 (As you cast this spell, an opponent of your choice may pay 3 life. If they don't, they fail the trial.)
When Steeple Deadeye enters, if the trial was failed, you gain 3 life and put a +1/+1 counter on Steeple Deadeye.

Hexproof
Cycling {3} ({3}, Discard this card: Draw a card.)

You may reveal the second card you draw each turn as you draw it. When you do, create a 2/2 blue Avatar Wizard creature token with prowess if it's a creature card. If it's a land card, create a Treasure token. If it's a noncreature, nonland card, Steward of Second Sphere can't be blocked this turn.
{3}{W/U}{W/U}: Scry 2.

When Still-Smoke Skirmisher dies, each opponent loses 1 life and you gain 1 life. Then return Still-Smoke Skirmisher to the battlefield at the beginning of the next end step if it was sacrificed.

Creature cards in your graveyard have revive. The revive cost is equal to their mana cost. (Pay that creature's mana cost, Exile that card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

When Stillmarsh Brinecatcher enters, return another target nonland permanent to its owner's hand.
At the beginning of your end step, if you control a permanent with mana value 5 or greater, you may sacrifice Stillmarsh Brinecatcher. If you do, draw a card and create a 3/2 colorless Horror creature token.

Menace
Whenever you discard a card, Stillsong Enforcer deals 1 damage to target opponent or planeswalker.

Breach (Whenever this deals combat damage to a player, you may have it become an Aura Curse until your next turn. Attach it to that player.)
At the beginning of your end step, each cursed opponent loses 1 life. You gain life equal to the life lost this way.

When Stilthorpe Falconer enters, create a 1/1 white Bird creature token with flying.

Menace (This creature can't be blocked except by two or more creatures.)
Sacrifice another creature: Stingbrood Warlord gains deathtouch until end of turn.

Stinging Swordfish has first strike as long as it's attacking.
When Stinging Swordfish dies, create an Equipment enchantment artifact token named Fish Sword with “Equipped creature gets +3/+1,” “Equipped creature has first strike as long as it's attacking,” and equip {2}. You may attach that token to a creature you control.

Flying
Whenever you cast your first spell each turn, add {U}{U}, {U}{R}, or {R}{R}.
Whenever you cast your second spell each turn, investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)
Whenever you cast your third spell each turn, Stirring Igmel deals 3 damage to each opponent.

Stoic Djinn has flying as long as it has a counter on it.

Stormbred Elemental gets +1/+1 as long as you control an Island.
{U}: Stormbred Elemental gains flying until end of turn.

Prowess
Pay 2 life: Change a target of target spell or ability to Straw Intellectual.

Stray Sighthound can't be blocked by creatures with power 2 or less.
Attain {3}{G} ({3}{G}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

When Strayseeker Ranger enters, search your library for a creature card with mana value 2 or less, reveal it, put it into your hand, then shuffle.
Recite {W}{W} (You may cast this creature from your graveyard for its recite cost. If you do, exile it when it enters or if it would leave the battlefield.)

Strider Engine enters tapped and with a -1/-1 counter on it unless you spent at least three colors of mana to cast it.

Whenever another Warrior creature you control attacks, you may return Stubborn Blademate from your graveyard to the battlefield tapped and attacking. If you do, sacrifice it at end of combat.

Improve {G} ({G}: 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.)
Whenever an Elf you control dies, you may search your library for a land card, reveal it, put it into your hand, then shuffle.