Confidence:StrongSolidPlausibleTentativeSpeculative

• Reanimate

  B

  SorceryB

  Reanimate puts River Merfolk onto the battlefield from a graveyard without casting it, so its mana cost is never paid.
• Helping Hand

  W

  SorceryW

  Helping Hand puts River Merfolk onto the battlefield from a graveyard without casting it, so its mana cost is never paid.
• Unearth

  B

  SorceryB

  Unearth puts River Merfolk onto the battlefield from a graveyard without casting it, so its mana cost is never paid.
• Grave Researcher // Reanimate

  2BB

  Creature — Troll Warlock // SorceryB

  Grave Researcher puts River Merfolk onto the battlefield from a graveyard without casting it, so its mana cost is never paid.
• Elrond, Moon-Reader

  2U

  Legendary Creature — Elf NobleU

  River Merfolk has activated abilities that aren't mana abilities, fueling Elrond, Moon-Reader's activation-trigger.
• Illusionist's Bracers

  2

  Artifact — EquipmentC

  River Merfolk has activated abilities that aren't mana abilities, fueling Illusionist's Bracers's activation-trigger.
• Rings of Brighthearth

  3

  ArtifactC

  River Merfolk has activated abilities that aren't mana abilities, fueling Rings of Brighthearth's activation-trigger.