msem-instigator

Siege Ent can't be blocked by creatures with power 2 or less as long as it's embattled. (A creature is embattled if it has a counter on it or a permanent you control attached to it.)

Flying
Other creatures you control enter with an additional +1/+1 counter on them.
If a spell or ability an opponent controls causes you to discard Sigarda, Dawn of Mercy, put it onto the battlefield instead of putting it into your graveyard.

Revive {3}{B} ({3}{B}, 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.)

Flash
Flying
Menagerie — Whenever Sigilwing Familiar deals combat damage to a player, if there are five or more creature types among creatures you control, draw a card.

Flying
When Silent Griffin enters, draw three cards, then put two cards from your hand on top of your library.

As Silent Hypnotist enters, choose a noncreature card name.
Spells with the chosen name can't be cast.
Activated abilities of sources with the chosen name can't be activated unless they're mana abilities.

Flash (You may cast this spell any time you could cast an instant.)
When Silk Crane Adept enters, target creature gains hexproof until end of turn.

Reach

Flying, vigilance
When Silksoul Celestial enters, search your library for an enchantment or Aurashaper card, reveal it, put it into your hand, then shuffle.
When Silksoul Celestial dies, you may put an enchantment or Aurashaper card from your hand or graveyard onto the battlefield.

Whenever Silt Lurker deals combat damage to a player, mill three cards, then draw a card if seven or more cards are in your graveyard.
Cycling {G} ({G}, Discard this card: Draw a card.)

Whenever Siltbottom Freebooters attacks, target Pirate gains menace until end of turn.

{2}{U}, {T}: Look at the top card of your library. If it's a Scout card or a card with an Adventure, you may reveal it and draw a card.

{2}{B/G}: Return Silthway Scuttler from your graveyard to your hand.
{2}{B/G}, Exile Silthway Scuttler from your graveyard: Put two +1/+1 counters on target creature.

Desecrate (When this creature enters, you may sacrifice an artifact or land.)
When Siltsplitter desecrates a permanent, each opponent sacrifices an artifact.

Silver-Dawn Elder gets +2/+2 as long as you control five or more lands.
Revive—{2}{G}{G}, Discard a card. ({2}{G}{G}, Discard a card, Exile this card from your graveyard: Create a token that's a copy of this card, except it has no mana cost. Activate this ability only any time you could cast a sorcery.)

First strike
Whenever Silverblade Cathar and at least two other creatures attack, draw a card.

At the beginning of combat on your turn, target creature you control gains vigilance until end of turn. That creature can't be blocked this turn.

When Silverhorn Tactician enters, permutate. (You may remove a counter from a creature you control. If you do, put two +1/+1 counters on a different creature you control.)

At the beginning of each end step, if you gained life this turn, you get a glory counter. (You lose glory counters before life. A player with ten or more glory counters wins the game.)

Whenever Sir Roger Wimbledon deals combat damage to a player, you may put an exiled creature card you own into your hand.

Flying
Sire of Empires gets +1/+1 as long as you control another creature, has lifelink as long as you control an enchantment, and has vigilance as long as you control an artifact.

Whenever Sirif of the Lord's Legion attacks, you may put a lore counter on target Saga you control.
Whenever Sirif deals combat damage to a player, you may remove a lore counter from target Saga you control.

Advance {3}{R} (Cast this for {3}{R} to advance it as it enters. {3}{R}: Advance this as a sorcery.)
Whenever another artifact creature you control enters or a permanent you control advances, Skalorian Orbguard deals 1 damage to any target.

Resolute — Skerry Serpent must be blocked if able as long as its power is greater than its base power.

Reach (This creature can block creatures with flying.)
Whenever a creature you control enters for the first time each turn, untap Skittering Swarm. It gets +1/+1 until end of turn.