msem-instigator

Creatures you control get +1/+1 until end of turn.
Artifact creatures and Elementals you control get +1/+1 until end of turn.

Enchant creature
Converge — When Big Enough enters, 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.

Trample, haste, indestructible
Whenever a permanent an opponent controls is put into a graveyard from the battlefield, put a +1/+1 counter on target Ooze.

When Biohazard enters, 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.

Return up to one target spell to its owner's hand.
Create two 1/1 blue Elemental creature tokens with flying. They can't block this turn.

Lifelink
At the beginning of your end step, if you've gained 3 or more life this turn, create a 2/2 white Bird creature token with flying.

Create a 1/1 blue Bird creature token with flying, then put the bottom card of your library into your hand.

Whenever Birthing Elemental or another nontoken creature you control dies, create a 1/1 green Saproling creature token.

Target player discards a card. Return a creature card from your graveyard to your hand.

Desecrate (When this creature enters, you may sacrifice an artifact or land.)
When Bitter Wanderer desecrates a permanent, each opponent loses 2 life and you gain 2 life.

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

You lose 1 life and exile the top card of your library. You may repeat this process any number of times. Then put up to two cards exiled this way into your hand.
Coalesce {5}{B}{B} (You may cast this from your graveyard for {5}{B}{B}. If you do, it enters as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)
Black Hole Musings's power is equal to the number of cards in exile.

Menace
When Blackmoat Bully dies, put a +1/+1 counter and a menace counter on target creature you control.

First strike, vigilance

Flash
When Blade of Mautakesh enters, 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.)

Flying
Whenever Blade of Reckoning deals combat damage to a player, that player sacrifices an untapped permanent.

Blade of the Many can be attached only to a nonlegendary creature.
Equipped creature gets +2/+2.
Each other creature you own is a copy of equipped creature.
Effects that would cause permanents you own to become creatures cause them to become copies of equipped creature instead.
Equip {6}

Invigorate X, where X is the number of creatures you control. (Choose a creature with the least power among creatures you control and put X +1/+1 counters on it.)

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Advance {1}{R}{W}{B} (Cast this for {1}{R}{W}{B} to advance it as it enters. {1}{R}{W}{B}: Advance this as a sorcery.)
Whenever Blastburn Angel advances, it gets +1/+1 and gains flying until end of turn. It deals 1 damage to each other creature.

Blazing Beam deals 4 damage to any target.

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

When this creature enters, create a 1/1 white Peasant creature token.
{W}, {T}: Tap target creature with power X or less, where X is the number of creatures you control.

Enchant creature
Enchanted creature gets +2/+2.
When enchanted creature dies, return it to the battlefield under its owner's control.

When Blessing from Wings on High enters, you may search your library for an Angel or Nephilim card, reveal it, put it into your hand, then shuffle.
Creatures you control get +1/+0.