Harbinger of the Seas

• WWizards of the Coast2024-06-07
  Official Ruling
  Nonbasic lands lose any other land types and abilities they had. They gain the land type Island and gain the ability ": Add ."
• WWizards of the Coast2024-06-07
  Official Ruling
  If a nonbasic land has an ability that causes it to enter the battlefield tapped, it will lose that ability before it can apply. The same is also true of any other abilities that modify how a land enters the battlefield or apply "as" a land enters the battlefield, such as that of Vesuva or Cavern of Souls.
• WWizards of the Coast2024-06-07
  Official Ruling
  If Harbinger of the Seas loses its abilities, it continues to turn nonbasic lands into Islands. This is because effects that change subtypes are applied before considering effects that remove abilities, regardless of the order in which those effects started.
• WWizards of the Coast2024-06-07
  Official Ruling
  Harbinger of the Seas doesn't affect names or supertypes. It won't turn any land into a basic land or remove the legendary supertype from a legendary land, and the lands won't be named "Island."
• WWizards of the Coast2024-06-07
  Official Ruling
  If a nonbasic land has an ability that triggers "when" it enters the battlefield, it will lose that ability before it can trigger.

Rulings via Scryfall

• WO
  Wizards of the Coast2026-02-09

  Harbinger of the Seas was unbanned in historic on February 9, 2026. We believe that Magus of the Moon and Harbinger of the Seas offer the right amount of interactivity against current and future land decks while inflicting minimum damage to the more casual Historic scene. These creatures are generally easier to remove than an enchantment like Blood Moon while being clear and effective options against nonbasic land-centric strategies. Source: https://magic.wizards.com/en/news/announcements/banned-and-restricted-february-9-2026
• WO
  Wizards of the Coast2024-06-03

  Harbinger of the Seas was banned in historic on June 3, 2024. We believe the other two cards, Winter Moon and Harbinger of the Seas, are above the power level threshold for Historic. Both cards affect the opponent's lands in a way we are cautious of in this format and will likely slow the game down. Source: https://magic.wizards.com/en/news/mtg-arena/mtg-arena-announcements-june-3-2024