msem-instigator

Equipped creature gets +2/+2.
{1}: Attach Scoundrel's Aetherlock to target Rogue.
Equip {2}

Scoundreleo enters with a +1/+1 counter on it if a creature you controlled dealt combat damage to a player this turn.

Double strike
Creatures you control have trample.
Excess noncombat damage that would be dealt by sources you control to permanents an opponent controls is dealt to that player instead.

Flying, haste
Ride {1}{R} ({1}{R}: Transform this and attach it to target creature you control without a Steed. Ride only as a sorcery.)

Flying, haste
Whenever Scourge of the Skies attacks, you may have it deal 5 damage to target creature.

Flying
Whenever you cast an instant or sorcery spell, Scourge of the Sun gets +1/+0 until end of turn. If you spent four or more mana to cast that spell, Scourge of the Sun gets an additional +1/+0 until end of turn.

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, discard a card. Then draw a card if an opponent is cursed.

Look at the top five cards of your library. You may reveal a Scout or land card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Reveal the bottom four cards of your library. Choose up to one permanent card with mana value 3 or less from among them and put it onto the battlefield. Put the rest on the bottom of your library in any order.

Look at the top four cards of your library. You may put a land card from among them onto the battlefield tapped, then put the rest into your graveyard.

Prevent all damage that would be dealt by creatures this turn.

Menace
Sacrifice another artifact: Scrap Buggy becomes an artifact creature until end of turn.
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.)

Scrap, the Second Myr has all activated abilities of other artifact creatures you control.

When Scrapgear Contraption dies, you may put a +1/+1 counter on target creature.

Vigilance, trample

Sacrifice Scrapyard Raiders: Target artifact gains indestructible until end of turn.

When Screams from the Deep enters, exile up to one target monocolored permanent.
As long as an opponent owns three or more cards in exile, you may have Screams from the Deep assign its combat damage as though it weren't blocked.
Evoke {5} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

Trample, prowess

Whenever you cast a spell with mana value greater than Script Jockey's power, put a +1/+1 counter on Script Jockey.

As Scrolls of Succession enters, choose a creature type.
Creatures you control of the chosen type get +1/+1.
{2}, {T}: Draw a card. Then discard a card unless you control three or more creatures of the chosen type.

Haste
Frenzy 2 (Whenever this creature attacks and isn't blocked, it gets +2/+0 until end of turn.)

When Scrying Pool enters, scry 2.
{1}, {T}: Add one mana of any color.

At the beginning of combat on your turn, if you cast both a creature spell and a noncreature spell this turn, untap up to one target land you control. It becomes a 3/1 red Sand creature with haste in addition to its other types until end of turn.

Enchant creature
Enchanted creature gets +2/+2.
Enchanted creature has hexproof as long as it's tapped.

When Scuripi Collector enters, return target artifact, enchantment, or land card from your graveyard to your hand.