msem-instigator

When Brawlhost Escapee dies, create a Function token with “Target creature you control fights target creature you don't control.” (It's an enchantment. As you cast an instant or sorcery spell, sacrifice any number of Functions to add their effects to that spell.)

Enchant creature you control
When Brawnsurge enters, enchanted creature deals damage equal to its power to up to one target creature an opponent controls.
Enchanted creature gets +1/+1 and has trample.

Alpha (You may have this creature enter with a +1/+1 counter on it. If you do, it becomes alpha and can only attack alone.)
Brazen Chimera can't be blocked except by three or more creatures as long as it has a +1/+1 counter on it.

Haste
Whenever one or more creatures attack, you may discard a card. If you do, draw a card.

Haste
At the beginning of your end step, sacrifice Breach Runner unless you return another creature you control to its owner's hand.

Counter target spell unless its controller discards two cards.

Destroy target artifact or tapped creature.

Rouse 2. Target permanent gains trample until end of turn if it's a creature. (Distribute two +1/+1 counters among permanents you control. If they aren't creatures, tap them and they become 0/0 creatures in addition to their other types.)

Enchant creature
Enchanted creature gets +2/+2 as long as it's an Actor. Otherwise, it gets -2/-2.

Until end of turn, target creature gets +2/+2 and gains trample and “Whenever this creature deals combat damage to a player, investigate.” (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)

Whenever Breakneck Barque becomes blocked by a creature, Breakneck Barque deals 3 damage to that creature.
Crew 3 (Tap any number of creatures you control with total power 3 or greater: This Vehicle becomes an artifact creature until end of turn.)

Exile target artifact or enchantment.

Breath of Hao'Tai deals 4 damage to any target.

{1}{G}: Target land you control becomes a 3/3 green Elemental creature until end of turn.
{1}{U}: Target enchantment you control becomes a 2/2 blue Nymph creature with flying until end of turn.

Exile target creature. You gain 2 life.

{T}: Add {C}.
{1}, {T}: Add one mana of any color. When that mana is spent to cast an Adventure spell, scry 1.

Flying

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

Choose a creature type. Return up to two target creature cards of the chosen type from your graveyard to your hand.

{T}: Add one mana of any color. Spend this mana only to cast an instant or sorcery spell.

When Briarform Apothecary enters, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)
Sacrifice a token: Briarform Apothecary gets +2/+2 until end of turn.

Hexproof
Whenever attackers are declared, the active player may pay {2} for each creature attacking you. Then you may have Bridge Kalak become an X/X Troll creature until end of turn, where X is the number of times that player didn't pay mana this way.

This spell costs {2} less to cast if you control a transformed permanent.
Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

When Bridled Narwhal enters, you gain 5 life.

Deathtouch
When Brig of the Damned enters, return target creature card from your graveyard to your hand. If that card's mana value is 2, you may return it to the battlefield instead.
Crew 2 (Tap any number of creatures you control with total power 2 or greater: This Vehicle becomes an artifact creature until end of turn.)