msem-instigator

When Sewer Dweller enters or dies, create a 1/1 black Rat creature token with “This creature can't block.”

[+1]: Up to two target creatures you control each get +1/+0 and gain first strike until end of turn.
[0]: You may sacrifice a creature. If you do, Shala Na, Urshadi Apostate deals 3 damage to any target.
[–3]: Discard your hand, then draw three cards.

When Shaman of Burning Ashes enters, return target instant or sorcery card from your graveyard to your hand. When you do, each opponent loses X life, where X is that card's mana value.

Destroy target artifact.

Equipped creature gets +1/+1.
Whenever equipped creature becomes the target of a spell or ability an opponent controls, you may return Shawl of Warding to its owner's hand. If you do, counter that spell or ability.
Equip {2}

Concord — {T}: Tap target nonland permanent. Activate only if there are four or more card types among permanents you control.

When this creature enters, draw a card, then put a creature you control on top of its owner's library.

Assault {2}{R}{G} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)
When Shemesh assaults, other creatures you control get +2/+2 and gain trample until end of turn.

Shepherd of the People can't be blocked by creatures with power 2 or less.
Whenever another Citizen or Peasant you control enters, Shepherd of the People gets +1/+1 until end of turn.

Whenever a land you control enters, you may tap target creature.

Kicker—Return a creature you control to its owner's hand. (You may return a creature you control to its owner's hand in addition to any other costs as you cast this spell.)
Counter target spell. If Shinrei's Ruse was kicked, draw a card.

Shock deals 2 damage to any target.

Equipped creature gets +1/+1.
Equip {1}

{T}: Add {C}.
Shrine of the World-Shepherd's power and toughness are each equal to the number of differently named lands you control.
{3}{G}: Shrine of the World-Shepherd becomes a Beast creature in addition to its other types until end of turn.

Flash
Harmony — Another target creature gets +1/+0 and gains indestructible until end of turn.
When Siamang Tale-Singer enters, harmonize. (Trigger up to one harmony of a permanent you control.)

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.)

When Sihari's Groundblazer enters, create a red Aura enchantment token named Searing Crags attached to target land. The token has enchant land, “Enchanted land's name is Charred Causeway” and “Enchanted land has {T}: This land deals 1 damage to each opponent.”

You may look at the top card of your library any time, and you may play Deserts from the top of your library.
[+1]: You gain 1 life. You may discard a card. If you do, draw a card.
[–2]: Sihari deals 2 damage to any target. You may play an additional land this turn.
[–4]: Create an X/X red and green Sand Elemental creature token with trample and haste, where X is the number of lands you control.

Silent Ridge enters tapped.
{T}: Add {B}.
Desertcycling {1}{B} ({1}{B}, Discard this card: Search your library for a Desert card, reveal it, and put it into your hand, then shuffle.)

When Silverhorn Battlemaster enters, put a +1/+1 counter on up to two target Warriors you control.
Whenever Silverhorn Battlemaster deals combat damage to a player, 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.)

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.

Double strike
Bushido 1 (Whenever this creature blocks or becomes blocked, it gets +1/+1 until end of turn.)
Whenever Sisters Suzakazi attacks, if you control another creature named Sisters Suzakazi, it deals 3 damage to any target.

Reach

This spell can't be countered.
{U}: Exile Skulkling. Return it to the battlefield under its owner's control at the beginning of the next end step.
{1}{U}: Until end of turn, Skulkling gets +1/-1 and can't be blocked.
{3}{U}: Draw a card. Create a 1/1 blue Rogue creature token.

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