msem-instigator

Target player sacrifices an attacking or blocking creature.

Sacrifice Chromeleon: Add one mana of any color. Scry 1.

Sacrifice a creature: Soulsorrow Shade gets +2/+2 until end of turn.

Reveal the top five cards of your library. You may put a permanent card revealed this way into your hand and the rest into your graveyard.

Whenever an opponent casts a spell during your turn, Hunter of Incantations deals 2 damage to that player.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Whenever you cast an enchantment, instant, or sorcery spell, Noble Barracuda gets +1/+0 until end of turn. Scry 1.

Trample
{4}{G}: Centaur Warband gets +2/+2 until end of turn.

This spell costs {1} less to cast for each +1/+1 counter among creatures you control.
Hoof Cliff Heartminer enters with four +1/+1 counters on it.
Whenever Hoof Cliff Heartminer or another creature you control leaves the battlefield, you may put its +1/+1 counters on another target creature you control.

Revive abilities of cards in your graveyard cost {1} less.
Sacrifice Fallow Grave: Create a 1/1 black Worm creature token.

Ward—Pay 3 life.
Igniderm enters with three ignition counters on it.
At the beginning of your upkeep, remove an ignition counter from Igniderm. If you can't, sacrifice it.

When Overgrowth Guardian enters, germinate. (Put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)

Whenever you cast a noncreature spell, Shiverpass Taskmage explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

When Viking Greataxe enters, attach it to target creature you control.
Equipped creature gets +2/+1.
Equip {3}

Equipped creature gets +2/+2 and has vigilance.
Equip {4}
Wanderlust — Wayfarer's Adornments has equip {0} as long as you control four or more differently named lands.

Trial 3 (As you cast this spell, an opponent of your choice may pay 3 life. If they don't, they fail the trial.)
When Heedless Adversary enters, you may have it deal 3 damage to target creature if that creature's controller failed the trial.

Siphon (Each -1/-1 counter you put on your creatures up to their remaining toughness while casting this pays for {1}.)
Target creature gets +2/+2 and gains indestructible until end of turn.

Create a token that's a copy of target permanent you control, except it isn't legendary. Then put Picture Perfect on the bottom of its owner's library if you cast it from exile.
Leitmotif (If you cast this spell from your hand, exile it as it resolves. When you next cast a spell that shares a color with this card, you may cast this from exile without paying its mana cost.)

Flash
Enchant player
When Curse of Doubt enters, counter target spell enchanted player controls unless they pay {2}.
{4}{U}: Enchanted player mills three cards.

Blooming Needles enters tapped unless you control another Desert.
{T}: Add {R} or {G}.

Whenever you cast a noncreature spell, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)

Erratic (When this creature dies, reveal the top four cards of your library. Put a creature card that costs less than this revealed this way onto the battlefield and the rest on the bottom of your library in any order.)
Chaosphere Ravager attacks each combat if able.

When Beyond the Veil enters, if it was cast, another target nonland permanent phases out. (Treat it and anything attached to it as though they don't exist.)
Nonland permanents you don't control with mana value 3 or less can't phase in.

Whenever a creature enters, it deals damage equal to its power to any target of its controller's choice.
When Interplanar Raid enters, you may aetherize target creature you control. (To aetherize, create a token that's a copy of it, then exile that token.)

Ferocious Flame deals 2 damage to any target. If you control a creature with power 4 or greater, it deals 4 damage instead.

Whenever you cast an instant or sorcery spell, put a +1/+1 counter on Kederekt Seawitch, then scry 1.