msem-instigator

You may put a creature card from your hand onto the battlefield. That creature gains haste until end of turn. Sacrifice that creature at the beginning of the next end step unless you pay its mana cost reduced by up to {3}{R}{R}.

Augury Hydra enters with X +1/+1 counters on it.
When Augury Hydra enters, scry Y, where Y is its power, then draw Y cards.

Menace, haste
At the beginning of combat on your turn, gain control of up to two target noncreature, nonland permanents your opponents control until end of turn. Untap those permanents. You may attach any number of Equipment among them to creatures you control until end of turn. (They unattach at end of turn.)

Whenever Aukashi, Electric Legacy enters or equipped creature dies, attach Aukashi to a creature you control.
Whenever equipped creature deals damage to a permanent or player, Aukashi deals 2 damage to that permanent or player.
Equipped creature has double strike as long as a creature you controlled died this turn.
Equip {2}

Whenever equipped creature deals combat damage to a player, exile an instant or sorcery card from your graveyard. Then copy any number of cards exiled with Aukashi, the Rising Storm with mana value X or less and cast the copies without paying their mana costs, where X is the number of cards exiled with Aukashi.
Equip {2}

When Aureate Golem enters, create a Treasure token. Each other player creates a tapped Treasure token. (They're artifacts with “{T}, Sacrifice this artifact: Add one mana of any color.”)

{1}, {T}, Sacrifice Aureate Panorama: Search your library for a land card that could produce each color of mana, put it onto the battlefield, then shuffle.

This spell can't be countered.
Hexproof

Menace
Whenever Auros, Flame-Cursed attacks, it deals 3 damage to each opponent.

This spell can't be countered.
When Auros enters, it deals damage to any target equal to your devotion to red. (Each {R} in the mana costs of permanents you control counts toward your devotion to red.)
{R}, Sacrifice Auros, Discard your hand: Draw three cards.

Trample
Auspicious Qilin must be blocked if able.

Once each turn, when you cast a noncreature spell, create a 1/1 colorless Human creature token. Put a prowess counter on it.

Whenever you play a land, return a card at random from your graveyard to your hand.
Whenever you cast a green spell, reveal the top card of your library. If it's a land card, put it into your hand. Otherwise, mill it.

When Automated Blademaster enters, create a colorless Equipment artifact token named Sword with “Equipped creature gets +1/+1” and equip {1} and attach it to Automated Blademaster.

Harmony — Scry 2.
{T}: Harmonize. Activate only as a sorcery. (Trigger up to one harmony of a permanent you control.)

Reach
When Automated Overwatch enters, create a Function token with “If this spell would create one or more tokens, it creates twice that many of those tokens instead.” (It's an enchantment. As you cast an instant or sorcery spell, sacrifice any number of Functions to add their effects to that spell.)

({T}: Add {R} or {G}.)
Autumn Canopy enters tapped.
{1}, {T}: Add {R}{R} or {G}{G}.

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

{T}: Add {C}.
{2}, {T}: Add three mana of any one color.

Creatures you control get +1/+0 and gain haste until end of turn.

Distribute four +1/+1 counters among up to two target lands you control. Those lands become 0/0 Elemental creatures with haste until end of turn. If you're attacking, tap those lands. They're attacking.
Showcase {3}{R}{G} (Then if you cast this for its showcase cost, create a 1/1 red Bard creature token that can't block and exile this. Whenever it attacks, you may cast a free copy of this.)

Autumnal Boscage enters tapped unless you control a basic land.
{T}: Add {R} or {G}.

Sacrifice a creature: Exile the top card of your library. Until end of turn, you may play that card.

Trample
Harmony — Avalanche Drummer deals 1 damage to any target.
When Avalanche Drummer enters, harmonize. (Trigger up to one harmony of a permanent you control.)

{2}: Add one mana of any color.