msem-instigator

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

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.

Batrest, Fellor Leader gets +1/+1 for each Equipment attached to it.
When Batrest enters, create Astrina, Forged for Glory, a legendary colorless Equipment artifact token with “Equipped creature gets +4/+4 and has first strike and lifelink” and equip {4}. Attach it to Batrest.

Alpha (You may have this creature enter with a +1/+1 counter on it. If you do, it becomes alpha and can only attack alone.)
Battering Swine attacks each combat if able.

Haste
Glorious end — When Battle-Blind Leonin dies, if it was tapped, you may pay {B}. If you do, you draw a card and you lose 1 life.

Deploy 2 (As this enters, you may tap any number of other creatures you control with total power 2 or more.)
When Battlefield Preserver enters deployed, exile target nonland permanent until Battlefield Preserver leaves the battlefield.

Lifelink (Damage dealt by this creature also causes you to gain that much life.)

Flying, lifelink

Deathtouch, lifelink
Whenever Batwing Initiate deals combat damage to a player, put a +1/+1 counter on it.
Pay 4 life: Batwing Initiate gains flying 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.

Beachhead Partisan has first strike as long as it's attacking.
When Beachhead Partisan enters, it deals X damage to any target, where X is the number of tapped creatures you control.

Bearer of Fables gets +1/+1 for each enchantment you control.
{2}{G}: Look at the top four cards of your library. You may reveal an enchantment card or a card with fabled from among them and put it into your hand. Put the rest on the bottom of your library in any order.

When Bearer of Silk Chains enters, gain control of target nonland permanent until Bearer of Silk Chains leaves the battlefield. If that permanent is a Horror, gain control of it instead.

When Bearer of Tormented Truths enters, look at target player's hand. You may exile a nonland card from among them. If you do, that player draws a card.
You may play cards exiled with Bearer of Tormented Truths, and you may spend mana as though it was mana of any color to cast them.

{2}, {T}: Tap target creature.

{T}: Add {G}.
{T}: Add {G}{G}. Activate only if you control an Elemental.

{2}{G}: Target face-down creature you control becomes a Beast with base power and toughness 5/5 until end of turn.

Whenever Beatrice's Dodger attacks, create a 1/1 black and red Human Rogue creature token that's tapped and attacking.
{3}{B}{R}: Return Beatrice's Dodger from your graveyard to the battlefield
tapped and transformed.

Whenever another creature you control dies, put a +1/+1 counter on Belligerent Mob.

{T}: Until end of turn, target land is every basic land type in addition to its other types and its name becomes Morphic Sea.

Armed — Beragnian Savage gets +1/+0 and has first strike as long as it's equipped.

When Bestower of Blooms dies, create an X/X green Plant Knight creature token, where X is Bestower of Blooms's power.