Assimilate Essence

Counter target creature or battle spell unless its controller pays {4}. If they do, you incubate 2. (Create an Incubator token with two +1/+1 counters on it and "{2}: Transform this artifact." It transforms into a 0/0 Phyrexian artifact creature.)

Oceanus Dragon

Flying
When Oceanus Dragon enters, tap target creature an opponent controls. Goad it. (Until your next turn, that creature attacks each combat if able and attacks a player other than you if able.)

Roil Elemental

Flying
Landfall — Whenever a land you control enters, you may gain control of target creature for as long as you control Roil Elemental.

Elminster's Simulacrum

For each opponent, you create a token that's a copy of up to one target creature that player controls.

Sea Gate Oracle

When Sea Gate Oracle enters, look at the top two cards of your library. Put one of them into your hand and the other on the bottom of your library.

Bane's Contingency

Counter target spell. If that spell targets a commander you control, instead counter that spell, scry 2, then draw a card.

Pseudodragon Familiar

Flying
{2}{U}: Target creature gains flying until end of turn.

Remove Soul

Wind Drake

Bubble Up

Put target instant or sorcery card from your graveyard on top of your library. (Then exile this card. You may cast the creature later from exile.)

Aquatic Alchemist

Whenever you cast your first instant or sorcery spell each turn, Aquatic Alchemist gets +2/+0 until end of turn.

Aquatic Alchemist // Bubble Up

Cathartic Adept

Island

Tidewater Minion

Defender (This creature can't attack.)
{4}: Tidewater Minion loses defender until end of turn.
{T}: Untap target permanent.

Naiad of Hidden Coves

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

Martyr of Frost

{2}, Reveal X blue cards from your hand, Sacrifice Martyr of Frost: Counter target spell unless its controller pays {X}.

Cancel

Aether Adept

When Aether Adept enters, return target creature to its owner's hand.

Hive Mind

Whenever a player casts an instant or sorcery spell, each other player copies that spell. Each of those players may choose new targets for their copy.

Windrider Eel

Flying
Landfall — Whenever a land you control enters, Windrider Eel gets +2/+2 until end of turn.

Illusionary Servant

Flying
When Illusionary Servant becomes the target of a spell or ability, sacrifice it.

Alora, Merry Thief

Whenever you attack, up to one target attacking creature can't be blocked this turn. Return that creature to its owner's hand at the beginning of the next end step.
Choose a Background (You can have a Background as a second commander.)

Jace Beleren

+2: Each player draws a card.
−1: Target player draws a card.
−10: Target player mills twenty cards.

Academy Loremaster

At the beginning of each player's draw step, that player may draw an additional card. If they do, spells they cast this turn cost {2} more to cast.

Negate

Unsummon

Return target creature to its owner's hand.

Disdainful Stroke

Counter target spell with mana value 4 or greater.

Sailors' Bane

This spell costs {1} less to cast for each card you own in exile and in your graveyard that's an instant card, a sorcery card, or a card that has an Adventure.
Ward {4} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {4}.)

Merchant of Secrets

When Merchant of Secrets enters, draw a card.

Sand Augury

Scry 1, then draw a card. (Then exile this card. You may cast the creature later from exile.)

Young Blue Dragon

Young Blue Dragon // Sand Augury

Summoner's Bane

Counter target creature spell. Create a 2/2 blue Illusion creature token.

Dragonborn Looter

{1}, {T}: Draw a card, then discard a card.

Phantasmal Abomination

Defender
When Phantasmal Abomination becomes the target of a spell or ability, sacrifice it.

Dream Fracture

Counter target spell. Its controller draws a card.
Draw a card.

Quest for Ancient Secrets

Whenever a card is put into your graveyard from anywhere, you may put a quest counter on Quest for Ancient Secrets.
Remove five quest counters from Quest for Ancient Secrets and sacrifice it: Target player shuffles their graveyard into their library.

Merfolk Observer

When Merfolk Observer enters, look at the top card of target player's library.

Baral, Chief of Compliance

Instant and sorcery spells you cast cost {1} less to cast.
Whenever a spell or ability you control counters a spell, you may draw a card. If you do, discard a card.

Phantom Warrior

Lay Bare

Counter target spell. Look at its controller's hand.

Conundrum Sphinx

Flying
Whenever Conundrum Sphinx attacks, each player chooses a card name. Then each player reveals the top card of their library. If the card a player revealed has the name they chose, that player puts it into their hand. If it doesn't, that player puts it on the bottom of their library.