msem-instigator

Other Knights and Soldiers you control get +1/+0.
Cowards can't block Knights and Soldiers you control.
When Belmord Bloodcurdler enters the battlefield, target creature an opponent controls becomes a Coward in addition to its other types.

Flying
When Benevolent Malach enters the battlefield, create a 1/1 colorless Construct artifact creature token.
Sacrifice Benevolent Malach: Artifacts you control gain indestructible until end of turn.

When Benign Whisper enters the battlefield, return another target creature to its owner's hand.

When Benthic Shipcrasher enters the battlefield, scry 3.

Armed — Beragnian Blacksmith can't attack unless it's equipped.
When Beragnian Blacksmith enters the battlefield, you may pay {2}. If you do, arm it. (Create a colorless Equipment artifact token with “Equipped creature gets +1/+0” and equip {2}, then attach it to this creature.)

When Bessie's Brood enters the battlefield, it fights target creature. (If there are no other creatures, it eats itself.)

When Beyond the Boneyard enters the battlefield, exile another target nonland permanent until Beyond the Boneyard leaves the battlefield.
Your opponents can't cast spells that share a card type with the exiled card during postcombat main phases.

Enchant creature
Converge — When Big Enough enters the battlefield, put X +1/+1 counters on enchanted creature, where X is number of colors of mana spent to cast Big Enough.
Enchanted creature has trample.

When Big Rock enters the battlefield, it deals 2 damage to any target.
{T}: Add {C}.
{T}, Sacrifice Big Rock: It deals 2 damage to any target. Activate this ability only if you control a Giant.

When Bind the Mind enters the battlefield, each poisoned opponent reveals their hand. Then choose a nonland card name.
Spells with the chosen name can't be cast.

Flash
When Binding Stones enters the battlefield, exile target creature with power 3 or less until Binding Stones leaves the battlefield.

When Biohazard enters the battlefield, destroy all other nonland permanents with mana value X or less.
At the beginning of each end step, each opponent loses life equal to the number of cards they've drawn this turn plus the number of spells they've cast this turn.

When Bird Call enters the battlefield, search your library for a creature card, reveal it, shuffle, then put that card on top.
{2}, {T}: Target creature gets +1/+1 until end of turn.

When Bitter Pact enters the battlefield, destroy target nonbasic permanent.
All permanents and players lose hexproof, indestructible, and protection and can't have or gain hexproof, indestructible, or protection.

When Bizarre Dreams enters the battlefield, destroy up to one target artifact or enchantment.
{3}{W}, Sacrifice Bizarre Dreams: Create two 1/1 blue Elemental creature tokens with flying. They can't block this turn.

When Bjarndyr Mender enters the battlefield, other creatures you control get +1/+1 and gain indestructible until end of turn.

When Bjarndyr Militant enters the battlefield, return target Aura card or card with fabled from your graveyard to your hand.

When Bjarndyr Outfitter enters the battlefield, arm up to two target creatures you control. (Create a colorless Equipment artifact token with “Equipped creature gets +1/+0” and equip {2}, then attach it to that creature.)

Lifelink
When Black Curtain Enforcer enters the battlefield, you may return target creature card with mana value 3 or less from your graveyard to your hand.

Flying
When Blackblood Feaster enters the battlefield, you may have each player sacrifice a creature.
Pay 2 life: Blackblood Feaster gets +2/+2 until end of turn. Activate only once each turn.

Flash
When Blade of Mautakesh enters the battlefield, creatures you control gain poisonous 1 until end of turn. (Whenever those creatures deal combat damage to a player, that player gets a poison counter. Multiple instances of poisonous trigger separately.)

When Blazeblade Sergeant enters the battlefield, adjudicate. (Create a white Aura enchantment token with enchant creature and “Sacrifice this token: Untap enchanted creature” attached to target creature.)
Whenever an Aura enters the battlefield under your control, target creature you control gains first strike until end of turn.

Trample, haste
When Blazing Fox enters the battlefield, you may tap an untapped creature you control. If you do, Blazing Fox gets +X/+0 until end of turn, where X is that creature's power.

Flying
When Bleak Condor enters the battlefield, mill four cards.

When Bleak Gorge Convoy enters the battlefield, you may search your library for a Desert card, put it onto the battlefield tapped, then shuffle.