msem-instigator

Flying
Wither (This deals damage to creatures in the form of -1/-1 counters.)

Whenever you cast an instant or sorcery spell, each opponent loses 1 life.
Attain {3}{B} ({3}{B}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

{B}: Revenant Shade gets +1/+1 until end of turn.

Whenever a creature dealt damage by Slave Child this turn dies, you may pay {B}. If you do, transform Slave Child.

Flying, lifelink
Morph {4}{B}
When Soul Cajoler is turned face up, manifest the bottom card of your library. (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)

Whenever Spirit of Infamy attacks, post a bounty on target creature. (Put a bounty counter on that creature. It has “When this creature dies, each opponent draws a card” as long as it has a bounty counter on it.)
Spirit of Infamy gets +2/+0 for each bounty counter on it.

Deathtouch
Whenever you cast a spell, you may pay 2 life. If you do, that spell gains deathtouch. (If it becomes a permanent, it keeps deathtouch indefinitely.)

Whenever you gain life, each opponent loses 1 life.

Flying
Whenever another creature you control enters from a graveyard or you revive a creature token, Tomb Scavenger gets +2/+0 until end of turn.

Deathtouch
Menagerie — When Venomfrill Familiar enters, if there are five or more creature types among creatures you control, you may destroy target creature or planeswalker.

Deathtouch
At the beginning of your end step, create a 1/1 black Rat creature token with “This creature can't block.”
{1}{B}, {T}, Sacrifice three other creatures: Each opponent discards a card and loses 2 life. You draw a card and gain 2 life.

Deathtouch

Menace, haste
As long as Arleigh, Spellblade Prodigy is tapped, the first spell you cast during each other player's turn costs {U/R} less to cast. This effect reduces only the amount of colored mana you pay. (For example, if you cast a spell with mana cost {1}{R}, it costs {1} to cast.)

Rhapsody {4}{R}{R} (Pay {4}{R}{R} to cast this as a 4/4, or as a sorcery to make it become a 4/4.)
Trample
Whenever Attuned Slayer deals combat damage to a player, exile that many cards from the top of your library. You may play those cards this turn.

Creatures you control that have been dealt damage this turn get +2/+0.
When Bloodspawn enters, it deals 1 damage to target creature you control.

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

Whenever one or more cards are put into an opponent's graveyard from their hand or library, Cackling Tormentor deals 2 damage to any target.

First strike, haste
Play with the top card of your library revealed.
{1}{R}: Darlene gets +1/+0 until end of turn.
Whenever Darlene deals damage to a player, if the top card of your library has mana value equal to the damage dealt, you may cast it without paying its mana cost.

{2}, {T}, Sacrifice Char Brewer: Exile target instant or sorcery card from your graveyard. You may cast that card this turn.

Whenever Current Shaper perseveres, it deals 1 damage to each opponent. (A creature perseveres when it survives damage.)

Whenever Dapper Ragamuffin attacks, it gets +2/+0 until end of turn if you control another attacking Pirate.

Mark (Whenever this creature attacks, mark target creature until end of turn.)
Whenever a marked creature dies, Diasune Tracker deals 1 damage to target player.

Wither (This deals damage to creatures in the form of -1/-1 counters.)
{X}{R}: Hearth Dweller gets +X/+0 until end of turn.

As a permanent you control explores an Aura, you may put it onto the battlefield attached to Inconspicuous Stowaway.
As long as Inconspicuous Stowaway is enchanted or equipped, she's a legendary 2/3 Cat Rogue named Agent C and has double strike.

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