msem-instigator

Whenever Borsuppa Roughrider attacks, create a colorless Desert land token named Scoured Path with “{T}: Add {R}.” Sacrifice that token at the beginning of the next end step.

Mark (Whenever this creature attacks, mark target creature until end of turn.)
Bounty Crusher has trample as long as it's being blocked by a marked creature.

Harmony — Create a colorless artifact token named Lotus Petal with “{T}, Sacrifice this artifact: Add one mana of any color.”
When Bouquet Singer enters, harmonize. (Trigger up to one harmony of one permanent you control.)

Whenever Bourasan Street Watch attacks, you may pay {W}. If you do, Bourasan Street Watch gains indestructible until end of turn.

Trample
{1}{G}: Brackish Kelpie gets +2/+2 until end of turn. Activate only once each turn.

Branch Prowler can't be blocked except by creatures with flying.

When Brand New Species enters, create a tapped 3/2 green Horror creature token.
Augmented — At the beginning of each end step, if you've spent {W}{U}{B}{R}{G} to cast spells this turn, put a +1/+1 counter on up to one target Horror you control.

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.

First strike
Other Rogues you control get +1/+0 and have first strike.
{2}, Sacrifice a permanent: Return target Rogue card from your graveyard to your hand.

When Breaker of Secrets enters, each player mills three cards from each of their libraries.
Recite {U} (You may cast this creature from your graveyard for its recite cost. If you do, exile it when it enters or if it would leave the battlefield.)

Breezerider has flying as long as you control a Nymph.
Breezerider has hexproof as long as you control another Monkey.

Flying, trample
Whenever Bretala of Redsky attacks, it gains double strike until end of turn unless defending player sacrifices a creature.

Prowess
Unveil {2}{U}{R} ({2}{U}{R}: Transform this. Unveil only as a sorcery.)

Flying, haste
At the beginning of the end step, sacrifice Brightflame Phoenix.
Retrace (You may cast this card from your graveyard by discarding a land card in addition to paying its other costs.)

Flash
When Brightsouled Guardian enters, return up to one target nonland permanent to its owner's hand.

Prowess, trample
Whenever Brilliant Diviner deals combat damage to a player, draw a card.

{2}{R}: Target creature can't block Brimstone Courser this turn.
Ride {3}{R}{R} ({3}{R}{R}: Transform this and attach it to target creature you control without a Steed. Ride only as a sorcery.)

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.)

Whenever you draw a card, you may have target player mill two cards.

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.

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

Reach
Hexproof (This creature can't be the target of spells or abilities your opponents control.)

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

Whenever another creature enters, Brutish Champion fights that creature.

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