Crystacean

Flash (You may cast this spell any time you could cast an instant.)

Ragged Veins

Flash
Enchant creature
Whenever enchanted creature is dealt damage, its controller loses that much life.

Naiad of Hidden Coves

During turns other than yours, spells you cast cost {1} less to cast.

Minion's Return

Flash
Enchant creature
When enchanted creature dies, return that card to the battlefield under your control.

Armor of Shadows

Until end of turn, target creature gets +1/+0 and gains indestructible. (Damage and effects that say "destroy" don't destroy it.)

Cunning Nightbonder

Flash
Spells with flash you cast cost {1} less to cast and can't be countered.

Feebleness

Flash
Enchant creature
Enchanted creature gets -2/-1.

Dimir Signet

Faerie Tauntings

Whenever you cast a spell during an opponent's turn, you may have each opponent lose 1 life.

Whisper Agent

Flash
When Whisper Agent enters, surveil 1. (Look at the top card of your library. You may put it into your graveyard.)

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Merfolk Trickster

Flash
When Merfolk Trickster enters, tap target creature an opponent controls. It loses all abilities until end of turn.

Mystic Subdual

Flash
Enchant creature
Enchanted creature gets -2/-0 and loses all abilities. (Mutating onto the creature won't give it new abilities. It can gain abilities in other ways.)

Slitherwisp

Flash
Whenever you cast another spell that has flash, you draw a card and each opponent loses 1 life.

Mutual Destruction

This spell has flash as long as you control a permanent with flash.
As an additional cost to cast this spell, sacrifice a creature.
Destroy target creature.

Fungal Fortitude

Flash
Enchant creature
Enchanted creature gets +2/+0.
When enchanted creature dies, return it to the battlefield tapped under its owner's control.

Stunning Strike

Flash
Enchant creature
When Stunning Strike enters, tap enchanted creature and remove it from combat.
As long as enchanted creature isn't legendary, it doesn't untap during its controller's untap step.

Spontaneous Mutation

Flash
Enchant creature
Enchanted creature gets -X/-0, where X is the number of cards in your graveyard.

Offalsnout

Flash
When Offalsnout leaves the battlefield, exile target card from a graveyard.
Evoke {B} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

Riptide Turtle

Grim Guardian

Constellation — Whenever Grim Guardian or another enchantment you control enters, each opponent loses 1 life.

Eel Umbra

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature gets +1/+1.
Umbra armor (If enchanted creature would be destroyed, instead remove all damage from it and destroy this Aura.)

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Portal Mage

Flash
When Portal Mage enters during the declare attackers step, you may reselect which player or permanent target attacking creature is attacking. (It can't attack its controller or their permanents.)

So Tiny

Flash
Enchant creature
Enchanted creature gets -2/-0. It gets -6/-0 instead as long as its controller has seven or more cards in their graveyard.

Lazotep Plating

Amass Zombies 1. (Put a +1/+1 counter on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)
You and permanents you control gain hexproof until end of turn. (You and they can't be the targets of spells or abilities your opponents control.)

Brinebarrow Intruder

Flash
When Brinebarrow Intruder enters, target creature an opponent controls gets -2/-0 until end of turn.

Soul Enervation

Flash
When Soul Enervation enters, target creature gets -4/-4 until end of turn.
Whenever one or more creature cards leave your graveyard, each opponent loses 1 life and you gain 1 life.

Eaten by Piranhas

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature loses all abilities and is a black Skeleton creature with base power and toughness 1/1. (It loses all other colors, card types, and creature types.)

Bladed Battle-Fan

Flash
When Bladed Battle-Fan enters, attach it to target creature you control. That creature gains indestructible until end of turn.
Equipped creature gets +1/+0.
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

Vexing Gull

Voidmage Husher

Flash (You may cast this spell any time you could cast an instant.)
When Voidmage Husher enters, counter target activated ability. (Mana abilities can't be targeted.)
Whenever you cast a spell, you may return Voidmage Husher to its owner's hand.

Fae Flight

Flash
Enchant creature
When Fae Flight enters, enchanted creature gains hexproof until end of turn.
Enchanted creature gets +1/+0 and has flying.

Tamiyo's Compleation

Flash
Enchant artifact, creature, or planeswalker
When Tamiyo's Compleation enters, tap enchanted permanent. If it's an Equipment, unattach it.
Enchanted permanent loses all abilities and doesn't untap during its controller's untap step.

Illusory Ambusher

Flash (You may cast this spell any time you could cast an instant.)
Whenever Illusory Ambusher is dealt damage, draw that many cards.