As long as you control a Jace planeswalker, Jace's Sentinel gets +1/+0 and can't be blocked.

[+1]: Whenever one or more creatures you control deal combat damage to a player this turn, draw a card, then discard a card.
[−2]: Create a 2/2 blue Illusion creature token with "When this creature becomes the target of a spell, sacrifice it."
[−5]: Create two tokens that are copies of Jace, Cunning Castaway, except they're not legendary.

[+1]: Draw a card.
[+1]: Untap all creatures you control.
[−9]: Gain control of up to three target creatures.

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
When Jade Guardian enters the battlefield, put a +1/+1 counter on target Merfolk you control.

: Put a +1/+1 counter on Jungle Delver.

Dinosaur spells you cast cost less to cast.

Flying
Creatures your opponents control enter the battlefield tapped.

Flying
When Kitesail Freebooter enters the battlefield, target opponent reveals their hand. You choose a noncreature, nonland card from it. Exile that card until Kitesail Freebooter leaves the battlefield.

Spells your opponents cast that target a Merfolk you control cost more to cast.
Abilities your opponents activate that target a Merfolk you control cost more to activate.

Kumena's Speaker gets +1/+1 as long as you control another Merfolk or an Island.

When Legion Conquistador enters the battlefield, you may search your library for any number of cards named Legion Conquistador, reveal them, put them into your hand, then shuffle your library.

Destroy target creature with power 4 or greater.

When Legion's Landing enters the battlefield, create a 1/1 white Vampire creature token with lifelink.
When you attack with three or more creatures, transform Legion's Landing.

Lightning Strike deals 3 damage to any target.

: Lightning-Rig Crew deals 1 damage to each opponent.
Whenever you cast a Pirate spell, untap Lightning-Rig Crew.

This spell costs less to cast if you control a Pirate.
Counter target spell unless its controller pays .

(Transforms from Dowsing Dagger.)
: Add three mana of any one color.

Whenever a creature you control explores, target creature an opponent controls gets -2/-2 until end of turn.

, Sacrifice an artifact or creature: Makeshift Munitions deals 1 damage to any target.