msem-instigator

Whenever Aspiring Hero attacks alone, put a +1/+1 counter on it.
When Aspiring Hero dies, each opponent gains life equal to its power.

Vanguard — Whenever a creature you control attacks alone, it gets +1/+1 until end of turn.

First strike
Chronicle (As this enters and after your draw step, add a lore counter. Stop after III.)
[Lore I-II] Put a +1/+1 counter on each of up to two other target creatures you control.
[Lore III] Remove all lore counters from Astral Capricine. Create two 0/1 white Goat creature tokens.

Menace
Chronicle (As this enters and after your draw step, add a lore counter. Stop after III.)
[Lore I-II] Exile the top card of your library. You may play that card this turn.
[Lore III] Remove all lore counters from
Astral Cerberus. It deals 3 damage to up to one target creature.

Deathtouch
Chronicle (As this enters and after your draw step, add a lore counter. Stop after III.)
[Lore I-II] You may sacrifice another creature. If you do, draw a card.
[Lore III] Remove all lore counters from Astral Leech. Each opponent sacrifices a creature.

Ward—Exile three cards from your graveyard. (Whenever this permanent becomes the target of a spell or ability an opponent controls, counter it unless that player exiles three cards from their graveyard.)
Menagerie — Astralhorn Familiar gets +2/+2 as long as there are five or more creature types among creatures you control.

When Astute Prophet enters, draw a card. If you control a legendary creature, draw two cards instead.

Menace
Whenever Aswaran Conquerer attacks, you may sacrifice a nonland permanent. If you do, creatures you control get +1/+1 and gain trample until end of turn.

Once each turn, when Atal, the Shroudmaker enters or you attack with one or more Horrors, you may create two tapped 1/1 colorless Human creature tokens. If Atal isn't a token, creatures you control gain flying until end of turn.
{2}{W}{W}, Discard Atal: Create a token that's a copy of Atal, the Shroudmaker, except it's 4/4 and isn't legendary. Activate only as a sorcery.

{2}: Atieh's Acolyte gains your choice of flying, vigilance, or lifelink until end of turn.
Whenever another Angel or Nephilim you control enters, put a +1/+1 counter on Atieh's Acolyte.
Surpass {4}{W}{U} ({4}{W}{U}: Transform this creature. This costs {1} less to activate for each counter on it. Activate only as a sorcery.)

Whenever Atieh's Destiny or another creature you control enters, you gain 1 life and scry 1.

Flying
Whenever Atieh's Disciple attacks, scry 1.

Prowess
Other Monks you control get +1/+1.
Whenever you cast a spell during another player's turn, untap each Monk you control.

Tribute 3 (As this creature enters, an opponent of your choice may put three +1/+1 counters on it.)
When Atuum Serpent enters, if tribute was paid, it gains defender. (This effect lasts indefinitely.)

{1}, Sacrifice a creature or enchantment: Exile the top card of your library. You may play that card this turn.

Flying, vigilance
When Augur of Pathways enters, surveil 1.

Flying
Whenever August Sunbird attacks, create a token that's a copy of another target creature you control, except it's 1/1, and it's tapped and attacking. Sacrifice the token at the beginning of the next end step.

This spell can't be countered.
Flying
When Aunor, Dragonsire or another Dragon you control enters, untap target land. That land becomes a 5/5 Dragon creature with flying and haste in addition to its other types.

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.”)

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.)

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

At the beginning of your end step, if you control two or more artifacts and/or enchantments, transform Avacyn's Partisan.

Flying, vigilance
At the beginning of your end step, return up to one target permanent card from your graveyard to the battlefield with an indestructible counter on it.

Flying
Spirited (When this creature enters, you may activate one of its activated abilities without paying its costs.)
{3}{U/B}{U/B}: Look at the top card of target opponent's library, then exile it face down. You may play that card for as long as it remains exiled, and you may spend mana as though it were mana of any type to cast that spell.

{2}: Avaricious Ghul gets +1/+1 until end of turn. If mana from a Treasure was spent to activate this ability, put a +1/+1 counter on Avaricious Ghul instead.