msem-instigator

{U}: Sea Strider gets +1/-1 until end of turn.
{U}: Sea Strider gets -1/+1 until end of turn.

Whenever Seasoned Lookout deals combat damage to a player, it voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)
Whenever Seasoned Lookout phases in, reveal the top card of your library. If it's a land card, put it onto the battlefield. Otherwise, put it into your hand.

Lifelink
Other creatures you control enter with an additional +1/+1 counter on them.
{2}{G}{W}: Nourish 1. (To nourish 1, either put a +1/+1 counter on this creature or create a 1/1 white Citizen creature token.)

Vigilance
Seawise Swain gets +1/+1 as long as you control an artifact.
Seawise Swain gets +1/+1 as long as you control an enchantment.

Defender (This creature can't attack.)
Adorned — Secluded Wildborn can attack as though it didn't have defender as long as it's equipped and/or enchanted.

Sabotage — Whenever an opponent or permanent an opponent controls becomes the target of a spell you control, target creature you control can't be blocked this turn.
{B}{B}{B}: Secret Thief gets +3/+0 until end of turn.

Whenever a creature you control enters, if it wasn't cast from your hand, you may mill a number of cards equal to its mana value. If you do, you may return a noncreature card milled this way from your graveyard to your hand.

Whenever you play a land, investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)
{1}{G}{U}{R}, Sacrifice three Clues: Exile cards from the top of your library until you exile three nonland cards. You may cast any number of those cards without paying their mana costs.

When Seeker's Guardian enters, reveal the top card of your library.

Flying
Whenever Seer of Clear Skies or another creature you control with flying attacks, that creature gets +1/+1 until end of turn.

Creatures with power less than Seer of the Hunt's power can't block it.

Experience (Whenever this creature attacks, if it has fewer than three +1/+1 counters on it, put a +1/+1 counter on it.)
Whenever a +1/+1 counter is put on Sejiri Skyhunter, it gains flying until end of turn.

Whenever a Survivor you control dies, you draw a card and you lose 1 life.

{3}: Sellsword Contraption becomes the creature type of your choice until end of turn.

Whenever a quest counter is put on a permanent you control, upgrade Selvar, the Seeker. (Give it one of the listed counters it doesn't already have.)
• +1/+1
• First strike
• Vigilance
• Reach
• Trample
• Lifelink

Death knell — Put a +1/+1, flying, lifelink, or menace counter on Senn, the Haunting Gaze, then trigger a death knell of another permanent you control. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Soldiercycling {1}{W}, Wizardcycling {1}{U}, Zombiecycling {1}{B}, Warriorcycling {1}{R}, Elfcycling {1}{G} ({1}{G}, Discard this card: Search your library for an Elf card, reveal it, and put it into your hand, then shuffle.)
Sentinel of Latter Days is every creature type as long as it's in your library.

Sentinel of the Highlands has hexproof as long as it's embattled. (A creature is embattled if it has a counter on it or has a permanent you control attached to it.)

Defender (This creature can't attack.)
Whenever you cast a spell, if it isn't your main phase, Serene Meditant loses defender until the beginning of your next end step.

Encourage ({T}: Another target creature you control gets +1/+1 until end of turn. Encourage only as a sorcery.)
Whenever Serul Cove Escort encourages a creature, that creature gains flying until end of turn.

When Servant of Precursors enters, you gain 4 life and draw a card.

Menace
Servitor Vanguard gets +X/+X, where X is the number of other artifacts you control.

Heroic — Whenever you cast a spell that targets Setessan Avenger, create a 1/1 green Nymph Dryad enchantment creature token with “{T}: Add {G}.”
Whenever Setessan Avenger deals combat damage to a player, untap all lands and Dryads you control.

Once each turn, when another creature you control enters, you may search your library for a card with the same name as that creature, reveal it, put it into your hand, then shuffle.