msem-instigator

When Azure Archaeologist enters, investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)
{2}, {T}, Sacrifice a Clue: Draw two cards, then discard a card.

Flying, trample
Prismatic — Whenever you cast a multicolored spell, put a +1/+1 counter on Azure Caterwauler.

Draw X cards, where X is the number of differently named lands you control.

When Azutahn Hydromancer enters, you may return target creature with power X or less to its owner's hand, where X is the number of differently named lands you control.

Whenever Babbling Experiment or another creature you control enters, target player mills two cards.

When Babyscale Bruiser enters, you may reveal a creature card from your hand. If you do, Babyscale Bruiser deals damage to target creature an opponent controls equal to the revealed card's power. That creature deals damage equal to its power to Babyscale Bruiser.

Put two +1/+1 counters on target creature. Untap that creature.

First strike
Back-Alley Gnoll has lifelink as long as it's enchanted by a black Aura.
Back-Alley Gnoll gets +2/+0 as long as it's enchanted by a red Aura.

Desecrate (When this creature enters, you may sacrifice an artifact or land.)
When Back-Alley Vandal desecrates a permanent, put a +1/+1 counter on Back-Alley Vandal.

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)

Mark (Whenever this creature attacks, mark target creature until end of turn.)
Whenever Backalley Silencer deals combat damage to a player, destroy target marked creature.

Haste
When Backdrop Artisan dies, audition for red. (Create a 1/1 Construct Actor creature token, a Warrior Role giving 3/2 and trample, or a Wizard Role giving 2/2 and prowess. Roles replace older ones.)

Target tapped creature gets -3/-3 until end of turn.

Backstabbing Courtiers can't attack unless you foretold a card or cast a foretold spell this turn.

Non-Actor creatures can't block this turn.

When Backstage Broken enters, create a 1/1 colorless Construct Actor creature token.
Whenever a token you control leaves the battlefield, you gain 1 life.

Deathtouch
When Backstage Comic enters, target player discards a card.

Lifelink
At the beginning of your end step, if you gained life this turn, put a +1/+1 counter on Backstage Manager.

Lifelink
Whenever Backstage Prompter deals combat damage to a player, put a +1/+1 counter on it.

When Backstage Tinkers enters, you may return an artifact card from your graveyard to your hand.

Whenever Backstreet Gutter-Runner attacks, if you control another Pirate or Rogue, it deals 2 damage to each opponent.

At the beginning of your end step, create a 1/1 white Soldier creature token.

When Backwoods Cultivator enters, put a +1/+1 counter on target creature you control.
When Backwoods Cultivator dies, proliferate. (You choose any number of permanents and/or players with counters on them, then give each another kind of a counter already there.)

Exile target creature or planeswalker. You gain 4 life.

Menace, haste