msem-instigator

Whenever Barker Duelist becomes blocked, defending player loses 1 life.
Unveil {3}{B}{R} ({3}{B}{R}: Transform this. Unveil only as a sorcery.)

Vanguard — Whenever a creature you control attacks alone, it gets +1/+3 until end of turn.

Whenever Barricade Watch is dealt damage, put a +1/+1 counter on it. (It must survive the damage to get the counter.)

Wither (This deals damage to creatures in the form of -1/-1 counters.)
Whenever Barrow Banshee deals combat damage to an opponent, that player discards a card at random.

{1}{U}: Target creature you control can't be blocked this turn by creatures with less power.
{1}{U}: Target creature you control can't be blocked this turn by creatures with greater power.

Flying
Chosen one ({W}{U}{B}{R}{G}: Transform this permanent.)

Flying, first strike, vigilance, lifelink
When Bastion Valkyrie dies, create three Function tokens with “Choose a creature targeted by this spell. It gains flying, first strike, vigilance, and lifelink until end of turn.” (They're enchantments. As you cast an instant or sorcery spell, sacrifice any number of Functions to add their effects to that spell.)

When Batiga Escort enters, create a 1/1 blue Bird creature token with flying.
{2}{W}, {T}: Put a +1/+1 counter on each creature you control with flying.

Each creature you control assigns combat damage equal to its toughness rather than its power.
Identity — Other creatures you control get +0/+1 as long as you control at least two other creatures that share a creature type with Battlewise Strategist.

When Bauble Collector enters, you may return another permanent you control to its owner's hand.

When Bearer of the Lantern enters, you gain 3 life.
Persist (When this creature dies, if it had no -1/-1 counters on it, return it to the battlefield under its owner's control with a -1/-1 counter on it.)

Desecrate (When this creature enters, you may sacrifice an artifact or land.)
When Beastback Raider desecrates a permanent, discard your hand, then draw three cards.

Concord — When Beetleback Forager enters, if there are four or more card types among permanents you control, return target card from your graveyard to your hand.

When Beguiling Witch enters, nourish 1. (To nourish 1, either put a +1/+1 counter on this creature or create a 1/1 white Citizen creature token.)
Whenever another creature dies, each opponent mills X cards, where X is Beguiling Witch's power.

Trample
Whenever Belligerent's Augur or another Warrior you control deals combat damage to a player, you may pay {1}{U}. If you do, draw a card.

Creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, Bellowing Thoctar gets +2/+2 until end of turn.

Armed — Beragnian Blacksmith can't attack unless it's equipped.
When Beragnian Blacksmith enters, 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.)

Whenever Beragnian Brawlmaster or another Warrior you control enters, that Warrior fights target creature an opponent controls.
{R}{W}: Target Warrior creature gets +1/+1 until end of turn.

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

At the beginning of your end step, mill two cards. Then if ten or more cards are in your graveyard, transform Bitter Duckling.

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

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

When Bjarndyr Outfitter enters, 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.)