Harbinger of the Seas

Bracket

• B1 Exhibition — not allowed: Mass land denial is not allowed
• B2 Core — not allowed: Mass land denial is not allowed
• B3 Upgraded — not allowed: Mass land denial is not allowed
• B4 Optimized — allowed: No restriction beyond the banned list
• B5 cEDH — allowed: No restriction beyond the banned list

Why

• Mass land denial
• Two-card combo with Choke, Curse of Marit Lage (early with Choke, Curse of Marit Lage)

How Lotus Vault estimates a bracket

Bracket: Lotus Vault analysis, with Commander Spellbook flags

Combos

2 known combos, 2 of them two-card.

• Harbinger of the Seas + Choke
  Produces: Nonbasic lands do not untap, Lock, Mass Land Denial
  View on Commander Spellbook
• Harbinger of the Seas + Curse of Marit Lage
  Produces: Nonbasic lands do not untap, Lock, Mass Land Denial
  View on Commander Spellbook

Combos: Commander Spellbook

Rulings

• 2024-06-07 · Wizards of the Coast
  Nonbasic lands lose any other land types and abilities they had. They gain the land type Island and gain the ability "{T}: Add {U}."
• 2024-06-07 · Wizards of the Coast
  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.
• 2024-06-07 · Wizards of the Coast
  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.
• 2024-06-07 · Wizards of the Coast
  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."
• 2024-06-07 · Wizards of the Coast
  If a nonbasic land has an ability that triggers "when" it enters the battlefield, it will lose that ability before it can trigger.

Rulings: Scryfall · Wizards of the Coast