msem-instigator

Flying, menace

Whenever Arisen Rider attacks, creatures your opponents control get -1/-1 until end of turn.
If Arisen Rider or another creature or planeswalker would die, exile it instead.

Trample
Whenever Arisen Tyrant attacks, mill three cards, then you may exile a card from your graveyard. You may cast a copy of the exiled card without paying its mana cost if the spell's mana value is 3 or less.
If Arisen Tyrant would die, exile it instead.

When Artillery Mage enters, it deals 5 damage to target creature you don't control.
{5}{R}, {T}, Discard a card: Artillery Mage deals 5 damage to target creature you don't control.

Menace (This creature can't be blocked except by two or more creatures.)
Whenever a creature you control deals combat damage to a player, you may have Aspect of Rage deal 1 damage to any target.

Indestructible
As long as your devotion to white and black is less than seven, Athreos isn't a creature.
Other creatures you control have lifelink.
Each player and cards in graveyards can't be the targets of spells or abilities your opponents 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.)

Flash
When Augmented Spawn enters, create a 1/1 blue Merfolk creature token for each Island you control.
{U}, Sacrifice a Merfolk: Augmented Spawn gains hexproof until end of turn.

This spell can't be countered.
Hexproof

Flying (This creature can't be blocked except by creatures with flying or reach.)
Whenever a nontoken creature you control enters, you may pay {1}{W}. If you do, create a 1/1 white Bird creature token with flying.

Trample
Whenever a player casts a noncreature spell, that player sacrifices a noncreature permanent.

Ayala, Shrouded Shepherd can't attack or block unless your life total is greater than or equal to your starting life total.
Lifelink, indestructible
Whenever a creature attacks you or a planeswalker you control, you may pay {2}. If you do, create a 1/1 white Spirit creature token with lifelink blocking that creature.

Flying
When Azos enters, each opponent reveals their hand. You choose any number of nonland cards in players' hands and/or creatures on the battlefield with total mana value 7 or less. Exile those cards and/or permanents.

Flying
{B/G}, Discard a creature card: Badru, the Skyshadow becomes a copy of the discarded card until end of turn, except he has flying and is 5/5.

{W}{B}{G}, Discard a card named Bahet, God of Destruction: Bahet gains indestructible and becomes a God creature in addition to its other types.
Whenever Bahet enters or attacks, each player chooses a number of lands they control equal to the number of lands controlled by the player who controls the fewest, then sacrifices the rest. Players discard cards and sacrifice non-God creatures the same way.

Defender (This creature can't attack.)
Discard a card: Barge Engulfer can attack as though it didn't have defender until end of turn.

When Batrest's Forgemaster enters, you may search your library for an Equipment card, reveal it, put it into your hand, then shuffle.
Whenever Batrest's Forgemaster attacks, Equipment spells you cast this turn cost {2} less to cast.

Whenever Battle-Scarred Combatant attacks or blocks, put a -1/-1 counter on it.

Creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, Bellowing Thoctar gets +2/+2 until end of turn.

When Benthic Tidebreaker enters, reveal the top card of each player's library and exile one of those cards. You may play that card for as long as it remains exiled, and you may spend blue mana as though it were mana of any color to cast it.
Evoke {1}{U} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

Armed — Whenever Beragnian Intimidator attacks, if it's equipped, target creature can't block this turn.

Lifelink
Whenever Bjarndyr Sergeant attacks, other creatures you control get +1/+1 and gain lifelink until end of turn.

Trample
Other Bears you control get +2/+2.

Blighted Unicorn can't block.
Ride {3}{B} ({3}{B}: Transform this and attach it to target creature you control without a Steed. Ride only as a sorcery.)