msem-instigator

Assault {R} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)
When Borsuppa Outrider assaults, it deals 2 damage to each opponent.

Menace (This creature can't be blocked except by two or more creatures.)
Assault {R} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)

When Boundary Hallowblade enters, create a white Aura enchantment token named Consecration Blessing attached to target land. The token has enchant land and “Enchanted land is named Hallowed Ground and has indestructible.”

Whenever a creature with a counter on it dies, create a Gold token. (It's an artifact with “Sacrifice this artifact: Add one mana of any color.”)

Other Hounds you control get +1/+1.

{1}{B}: Regenerate Brainless Ghoul. (The next time this creature would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)

First strike
Whenever you cast an instant or sorcery spell, if Branch Breaker dealt combat damage to a player this turn, you may sacrifice Branch Breaker. If you do, copy that spell. You may choose new targets for the copy.

Erratic (When this creature dies, reveal the top four cards of your library. Put a creature card that costs less than this revealed this way onto the battlefield and the rest on the bottom of your library in any order.)

Disgraced 2 (This enters with two -1/-1 counters. Whenever it deals combat damage to a player, remove all -1/-1 counters from it.)
When Branded Contractor dies, you may put its counters on target creature.

{5}: Brass Dragon gets +3/+0 and gains flying and first strike until end of turn.

When Brawlhost Escapee dies, create a Function token with “Target creature you control fights target creature you don't control.” (It's an enchantment. As you cast an instant or sorcery spell, sacrifice any number of Functions to add their effects to that spell.)

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.)
Brazen Chimera can't be blocked except by three or more creatures as long as it has a +1/+1 counter on it.

Other creatures you control have haste.
Whenever Breach Titan enters or attacks, abound. (To abound, you may cast a spell that costs less than this from your hand without paying its mana cost.)
At the beginning of your end step, return another target creature you control to its owner's hand.

Flying

If you would draw a card, you may instead choose creature or land and exile cards from the top of your library until you exile a card of the chosen kind. Put that card into your hand and put all other cards exiled this way on the bottom of your library in a random order.

Deathtouch
When Brig of the Damned enters, return target creature card from your graveyard to your hand. If that card's mana value is 2, you may return it to the battlefield instead.
Crew 2 (Tap any number of creatures you control with total power 2 or greater: This Vehicle becomes an artifact creature until end of turn.)

Augmented — Whenever Bril, the Oltenen Emissary enters or attacks, if you've spent {W}{U}{B}{R}{G} to cast spells this turn, destroy target permanent.

{T}: Brimstone Grenadiers deals 1 damage to target player.
Whenever you cast an instant or sorcery spell, untap Brimstone Grenadiers.

Defender
When Brine Duelist enters, create a Treasure token. (It's an artifact with “{T}, Sacrifice this permanent: Add one mana of any color.”)
{W}{U}{B}{R}{G}: Put a +1/+1 counter on Brine Duelist. It loses defender.

Double strike
Whenever you cast a noncreature spell, you may choose one that hasn't been chosen —
• Tap or untap target permanent.
• Draw two cards, then discard a card.
• Creatures you control get +1/+1 until end of turn.

{6}{U}: Brume Trench-Thrasher can't be blocked this turn. This ability costs {1} less to activate for each instant and sorcery card you own in exile and in your graveyard.

Ward {2}
Whenever Brunig enters or attacks, you may shuffle a card you own from outside the game into your library. If you do, you gain 2 life.
Brunig gets +2/+2 and has trample as long as your library contains at least forty cards more than the minimum deck size.

Whenever a creature you control becomes the target of a spell or ability, counter it unless its controller pays {1}. Whenever they do, put a +1/+1 counter on Brushcover Elf. (This includes your spells and abilities.)

Whenever another creature enters, Brutish Champion fights that creature.