msem-instigator

Menace (This creature can't be blocked except by two or more creatures.)
Avatar of Basat can't block.

First strike
When Avaz Cataphract enters, judge another target creature you control. (Honor it by putting a +1/+1 counter on it or condemn it by sacrificing it to draw a card.)

Flying
When Aven Strategist enters, return up to one target creature to its owner's hand.

Trample
Whenever a player casts a noncreature spell, that player sacrifices a noncreature permanent.

You may exert Awoken Elemental as it attacks. When you do, Awoken Elemental fights another target creature. (An exerted creature won't untap during your next untap step.)

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.

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.

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.)

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.

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

Whenever you create one or more tokens, put a +1/+1 counter on target creature you control.
{2}{G}{W}: Create a 0/1 green Plant creature token.

Prowess
Whenever Balefire Conjurer attacks, you may pay {1}{R}. If you do, exile the top card of your library. You may play that card until the end of your next turn.

Menace
As long as Balefire Harbinger is attacking, creatures in its pack can't be blocked unless defending player pays {1} and 1 life for each creature they control that's blocking one of those creatures. (Two or more creatures you control with equal power are in a pack.)

Haste (This creature can attack and {T} as soon as it comes under your control.)
{1}{U}: Balloon Dragoon gains flying until end of turn.

When Banished Prophet enters, you may remove a counter from a permanent you control. If you do, put a +1/+1 counter on another target creature you control.

When Banishing Initiate enters, you may exile target nonland permanent you don't control until Banishing Initiate leaves the battlefield.
At the beginning of your end step, sacrifice Banishing Initiate if a card exiled with it has mana value 3 or greater.

When Barbarian Hoarders enters, you may attach target Equipment you control to it.

Battalion — Whenever Barbaric Ambusher and at least two other creatures attack, you may tap target creature.

Barbed Reef enters tapped. When it does, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this, then you may put that card into your graveyard.)
{T}: Add {B}.
{4}{B}: Barbed Reef becomes a Jellyfish creature. (This effect lasts indefinitely.)

When Barricade Patrol enters, create a 1/1 white Human Soldier creature token.
Whenever another creature you control dies, Barricade Patrol deals 1 damage to target player.

When Basalt Ritualist enters, you may sacrifice a land or exile a land card from your graveyard. When you do, exile target creature, instant, or sorcery card from your graveyard. Until the end of your next turn, you may play that card. If that spell would be put into a graveyard, exile it instead.

Menace
Cat creatures you control have menace.
When Basat Priest enters, create a 2/2 white Cat creature token.

When Bastion Warden enters, exile target creature an opponent control until target enchantment you control leaves the battlefield.