Cheapest priced copy (Modern Horizons 3): $1.99 at Card Kingdom, near mint, their price list as of .
Harbinger of the Seas 
Creature — Merfolk Wizard · 2/2 · Mythic · Marvel Universe · #55
Nonbasic lands are Islands.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
4 printings · $1.99 to $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Marvel Universe | 55 | Mythic | $2.49 | $5.99 |
| Modern Horizons 3 | 63 | Rare | $1.99 | $2.99 |
| Modern Horizons 3 | 451 | Rare | $3.49 | $11.99 |
| Modern Horizons 3 Promos | 63s | Rare | — | $8.49 |
5 rulings from Wizards of the Coast
- 7 June 2024Nonbasic lands lose any other land types and abilities they had. They gain the land type Island and gain the ability "{T}: Add {U}."
- 7 June 2024If 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.
- 7 June 2024If 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.
- 7 June 2024Harbinger 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."
- 7 June 2024If a nonbasic land has an ability that triggers "when" it enters the battlefield, it will lose that ability before it can trigger.