msem-instigator

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.)

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.

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

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.

If you would draw a card, you may instead choose creature or land and exile cards from the top of your library until you exile a card of the chosen kind. Put that card into your hand and put all other cards exiled this way on the bottom of your library in a random order.

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.

{T}: Add {C}.
{1}, {T}: Add one mana of any color. This ability costs {1} less to activate if you control three or more basic lands.

Improve {1} ({1}: Put a +1/+1 counter on this creature. This costs {1} more to activate for each +1/+1 counter on it. Improve only as a sorcery.)
When Bridgehaven's Finest enters, create a 1/1 colorless Human creature token.
Other creatures you control get +1/+1 as long as Bridgehaven's Finest has two or more counters on it.

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.)

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

Shuffle your hand and graveyard into your library, then draw seven cards. You may have your life total become 10. You may cast a spell from your hand without paying its mana cost. Exile Bright Future.

Lifelink
When Bright-Eyed Adventurer enters, rediscover an enchantment card. (Look at the top card of your library. You may reveal it and put it into your hand if it's an enchantment card. Otherwise, put it back or into your graveyard.)

Spells cost {1} more to cast for each colored mana symbol in their mana cost beyond the first. (For example, a spell with mana cost {W}{W} would cost {1}{W}{W} to cast.)