msem-instigator

Creatures you don't control get -1/-0 as long as their controller is cursed.

{2}{U}{B}: Return Depth Charger to its owner's hand. If you do, return target creature to its owner's hand. That player discards a card.

Flash

Hexproof
Rapport — Whenever Depths Stalker and at least one other creature you control become tapped, Depths Stalker gets +1/+0 until end of turn and can't be blocked this turn.

Trample
Creatures your opponents control get -1/-0.

Experience (Whenever this creature attacks, if it has fewer than three +1/+1 counters on it, put a +1/+1 counter on it.)
When Determined Koi has three or more +1/+1 counters on it, remove them all and transform it.

Flash
When Devious Naiad enters, exchange control of it and target creature until end of turn. Untap that creature. It gains haste until end of turn.

Hexproof

Concord — When Dolphin Rider enters, if there are four or more card types among permanents you control, you gain 5 life.

This spell costs {1} less to cast for each card put into your graveyard from anywhere this turn.
At the beginning of each player's upkeep, that player loses 1 life, discards a card, and sacrifices a creature.

When Driftpicker enters, you may return another non-Crab creature you control to its owner's hand. If you do, create two 1/1 white Bird creature tokens with flying.

Whenever you cast a spell during an opponent's turn, creatures you control get +0/+1 until end of turn.

When Drownyard Scavenger enters, each player may return an instant or sorcery card from their graveyard to their hand.

When Dualglaive Acolyte enters, create a green Aura enchantment token named Calamighty attached to target creature. The token has enchant creature and “Enchanted creature gets +1/+1 and can't be blocked by creatures with power 2 or less.”

Vigilance
Whenever an opponent casts a spell, create a Treasure token unless that player pays {1}, then create a 1/1 white Merfolk creature token unless that player pays {1}.
{W}{U}{B}{R}{G}, {T}: Other creatures you control get +3/+3 and gain vigilance and indestructible until end of turn.

{3}{R}: Target creature can't be blocked this turn if it's an artifact creature or has power 2 or less.

{T}, Sacrifice an artifact or creature: Each opponent loses 1 life and you gain 1 life.

Menace
Whenever a creature you control deals combat damage to a player, exile the top card of that player's library. They lose life equal to that card's mana value.

Vigilance, ward {2}
{1}{G}, {T}: Put a +1/+1 counter on Dunetreader Tortoise. Activate only as a sorcery.

When Dusk Lieutenant enters, forebode. (Create a tapped colorless Omen artifact token with “{T}, Discard a card, Sacrifice this artifact: Draw a card.”)
Once each turn, when you discard a card during your turn, each opponent discards a card.

{4}{G}{G}: Eager Voyager voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)
Whenever Eager Voyager phases in, if it has no +1/+1 counters on it, put four +1/+1 counters on it.

When Edina's Soulsinger enters, create a 1/1 blue Jellyfish enchantment creature token with slipstream. (It can't be blocked as long as only creatures with slipstream are attacking.)

Whenever you cast an artifact or enchantment spell, scry 1, then create a 1/1 white Merfolk creature token.
Whenever you cast a Merfolk spell, look at the top card of your library. If it's an artifact or enchantment card, you may reveal it and put it into your hand.

When Elder Prophesier enters, look at the top five cards of your library. You may reveal a Curse card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Emissary of Dai Gui enters tapped.
Emissary of Dai Gui doesn't untap during your untap step unless it's in a pack. (Two or more creatures you control with equal power are in a pack.)
Creature spells you cast cost {1} less to cast.