msem-instigator

Whenever Brine Kelpie attacks, you may tap target creature an opponent controls.
Ride {2}{U} ({2}{U}: Transform this and attach it to target creature you control without a Steed. Ride only as a sorcery.)

Bristling Skoki has first strike as long as it's attacking.
Whenever Bristling Skoki blocks a creature, you may have it deal damage equal to its toughness to that creature.

When Broadside Raider enters, create a 1/1 blue and red Pirate creature token with haste.

Flash
Whenever you cast your second spell each turn, put a +1/+1 counter on Brooding Replacement.

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.)

Glorious end — When Brutal Champion dies, if it was tapped, each opponent sacrifices a creature.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Concord — As long as there are four or more card types among permanents you control, Bubbledome Waveblade gets +1/+1.

Whenever you cast a spell for its showcase cost, create a 2/2 blue Drake creature token with flying.

{3}{U}, {T}: Copy target instant or sorcery spell you control. You may choose new targets for the copy.

When Bug Catcher enters, create a 1/1 green Insect creature token.
Whenever a nontoken Insect you control dies, create a 1/1 green Insect creature token.

Double strike, trample
Arise {3}{R}{W} ({3}{R}{W}, Exile this from your graveyard as a sorcery: Create a black token copy of it that's also a Zombie and can't block.)
When Bugrund arises, you may put an Equipment card from your hand or graveyard onto the battlefield attached to it.

First strike
Ascend {1}{B} ({1}{B}: Exile this creature, then return it to the battlefield transformed. Ascend only as a sorcery.)

When Burglar Cat enters, exile the top card of target opponent's library. You may play that card during any turn you attacked with a Rogue, and you may spend mana as though it were mana of any color to cast it.

Haste
Sacrifice another creature: Bygone Aristocrat gains indestructible until end of turn.

Flying
When Byte Bug enters, conjure a card named Data-Point into your hand. (It's a sorcery that costs {2} with “Draw a card.”)

Menace, haste
Cadence (Whenever you cast a spell, if its mana value is greater than this creature's power, put a +1/+1 counter on this creature.)

Whenever you investigate for the first time each turn, investigate an additional time.
{T}, Sacrifice an artifact: Cadalon Inspector deals 2 damage to any target.

Artifact spells you cast cost {1} less to cast.
The first ability of an artifact you activate each turn costs {1} less.

Flying
When Cadre Hawk enters, you may search your library and/or graveyard for a card named Cadre Hawk, reveal it, and put it into your hand. If you searched a library this way, shuffle.

Whenever Calamity Scholar attacks, if you control an artifact, put a +1/+1 counter on it.

First strike
Whenever you make first contact, Callous Conduit deals 1 damage to target player or planeswalker. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)

{T}, Sacrifice Camahueto Calf: Return target land card from your graveyard to the battlefield.

Flying
Whenever Capricious Wind's Disciple attacks, another target creature in its pack gains double strike until end of turn. (Two or more creatures you control with equal power are in a pack.)

When Captured Mind dies, create a 3/3 white Construct enchantment creature token.

Haste
When Caravan Engulfer enters, creatures you don't control can't block this turn.