Harbinger of the Seas

What Does That Mean

This creature turns all nonbasic lands (lands that aren't Plains, Island, Swamp, Mountain, or Forest) into Islands.

This means any special lands your opponents control—like dual lands, utility lands, or mana-producing lands—become basic Islands, stripping away their special abilities while still providing blue mana.

Why Is That Important

You'd include this in a blue-heavy deck that wants to disrupt opponents' mana bases and color requirements, especially in formats where nonbasic lands are prevalent (like Commander or constructed formats with fetch lands and dual lands).

It's particularly effective against greedy multicolor decks that rely on specific color-fixing lands.

Paired with other blue control or tempo strategies, it can lock opponents out of their color requirements while being a reasonably costed threat that demands an answer.