msem-instigator

Whenever a creature you control attacks, you may sacrifice a Treasure. If you do, that creature can't be blocked this turn.

Wanderlust — When Transnaturalist enters, if you control four or more differently named lands, you may return target creature to its owner's hand.

Triumvirate Infiltrator has flying as long as you control another Elemental.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)

Ungoverned Sneak can't be blocked by creatures with greater power.
Attain {3}{U} ({3}{U}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

Flying (This creature can't be blocked except by creatures with flying or reach.)
Vaporkin can block only creatures with flying.

When Vedalken Machinist enters, create a 1/1 colorless Thopter artifact creature token with flying.
At the beginning of your upkeep, if you control three or more artifacts, draw a card.

Whenever you cast your second spell each turn, create a 1/1 blue Jellyfish Spirit creature token with flying.

Changeling (This card is every creature type.)
{1}{G/U}: Vune, the Facemaker becomes a copy of target creature you control, except its name is Vune, the Facemaker, it's legendary, and it has changeling and this ability. Activate only as a sorcery.

When Warden of the Archives enters, if you cast it, tap up to four target permanents.

Changeling (This card is every creature type.)
Watcher of the Polyps has all names of creature cards in addition to its name as long as it's on the battlefield.
Watcher of the Polyps gets +X/+X, where X is the greatest number of creatures you control with the same name.

Each other creature you control that's a Merfolk or an Elemental gets +1/+1.
Whenever you cast an instant, sorcery, or Merfolk spell, you may pay {1}. If you do, create a 1/0 blue Elemental creature token.

Flying
Welkin Tern can block only creatures with flying.

Flash (You may cast this spell any time you could cast an instant.)
When Wind-Blessed Guardian enters, you may return target permanent you control to its owner's hand.

When Wondering Oaf dies, return target instant or sorcery card from your graveyard to your hand.

At the beginning of your end step, if you're horrific, you draw a card and you lose life equal to the number of cards in your hand. (You're horrific as long as you've sacrificed a permanent or discarded a card this turn.)

{B}, {T}, Discard a card: Search your library for a card, then shuffle and put that card on top. You lose 2 life.

Spells you cast from anywhere other than your hand cost {1} less to cast.
Whenever an opponent casts a spell with the same name as a card they own in exile or in their graveyard, that player loses 1 life and you gain 1 life.

Flying, lifelink
Improve {1}{W/B} ({1}{W/B}: 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.)
Once each turn, when you gain life, you may have each opponent lose 2 life.

When Arisen Troops dies, you may destroy target creature that was dealt damage this turn.

Winnow (Damage dealt to this creature also causes you to lose that much life.)
{2}{B}: Each creature blocking Arroyo's Servant gains winnow until end of turn.

When Bayou Courier enters, you may pay 2 life. If you do, create a 3/2 green Crocodile creature token with trample.

When Blacksite Moderator enters, you may pay 2 life or sacrifice a permanent. If you do, draw a card.

{1}{B}: Blightcaller Shaman gains deathtouch until end of turn.

Flying (This creature can't be blocked except by creatures with flying or reach.)
When Blood-Tithe Harpy enters, target player loses 1 life and you gain 1 life.