Dandân

Dandân can't attack unless defending player controls an Island.
When you control no Islands, sacrifice Dandân.

Merfolk of the Pearl Trident

Old Man of the Sea

You may choose not to untap Old Man of the Sea during your untap step.
{T}: Gain control of target creature with power less than or equal to Old Man of the Sea's power for as long as Old Man of the Sea remains tapped and that creature's power remains less than or equal to Old Man of the Sea's power.

Boomerang

Return target permanent to its owner's hand.

Unsummon

Return target creature to its owner's hand.

Psionic Blast

Psionic Blast deals 4 damage to any target and 2 damage to you.

Island

Unstable Mutation

Enchant creature
Enchanted creature gets +3/+3.
At the beginning of the upkeep of enchanted creature's controller, put a -1/-1 counter on that creature.

River Merfolk

{U}: River Merfolk gains mountainwalk until end of turn. (It can't be blocked as long as defending player controls a Mountain.)

Strip Mine

{T}: Add {C}.
{T}, Sacrifice Strip Mine: Destroy target land.

Clone

You may have Clone enter as a copy of any creature on the battlefield.

Control Magic

Enchant creature
You control enchanted creature.

Force Spike

Counter target spell unless its controller pays {1}.

Braingeyser

Spell Blast

Counter target spell with mana value X. (For example, if that spell's mana cost is {3}{U}{U}, X is 5.)

Lord of Atlantis

Other Merfolk get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Tolaria

{T}: Add {U}.
{T}: Target creature loses banding and all "bands with other" abilities until end of turn. Activate only during any upkeep step.

Counterspell

Mana Drain

Counter target spell. At the beginning of your next main phase, add an amount of {C} equal to that spell's mana value.

Mishra's Factory

{T}: Add {C}.
{1}: Mishra's Factory becomes a 2/2 Assembly-Worker artifact creature until end of turn. It's still a land.
{T}: Target Assembly-Worker creature gets +1/+1 until end of turn.

Phantasmal Terrain

Enchant land
As Phantasmal Terrain enters, choose a basic land type.
Enchanted land is the chosen type.