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." If Harbinger of the Seas loses its abilities, it continues to turn nonbasic lands into Islands

Abigale, Poet Laureate Afterlife Insurance Agent of Masks Angel of Despair Anguished Unmaking Aron, Benalia's Ruin Arvad the Cursed Aryel, Knight of Windgrace Ascent of the Worthy Baron Bertram Graywater Barrowin of Clan Undurr Bartolom del Presidio Basilica Bell-Haunt Batwing Brume Black Widow, Double Agent Bleeding Effect Blood-Cursed Knight Bloodied Ghost Call to the Feast Campaign of Vengeance Cartel Aristocrat Cauldron Haze Chief of the Edge Chief of the Scale Cleric of Life's Bond Cliffhaven Vampire Closing Statement Conjurer's Ban Consecrate // Consume Consecrate // Consume Corpse Knight Cruel Celebrant Death Grasp Debt to the Deathless Denethor, Ruling Steward Despark Dire Tactics Drana's Emissary Dune Drifter Edgewalker Elas il-Kor, Sadistic Pilgrim Embalmed Ascendant Etchings of the Chosen Feast of the Victorious Dead Fiendish Panda Firja, Judge of Valor Fleshtaker Fracture General's Enforcer Gerrard's Verdict Gift of Orzhova Glasswing Grace Graceful Restoration Gwyllion Hedge-Mage Hardened Tactician Hei Bai, Spirit of Balance Hero of the Dunes Hidden Stockpile Hideous Fleshwheeler Holy Frazzle-Cannon Humiliate Indulging Patrician Invasion of New Capenna Judge Magister Gabranth Karai's Technique Karai, Future of the Foot Kaya, Bane of the Dead Killian's Confidence Killian, Ink Duelist Knight of the Last Breath Knights of the Black Rose Legion Lieutenant

Theyre often red creatures, black creatures, blue creatures, or some combination therein
After all the customer pays extra to get this version of the card
Hitting for 6 and paying 2 mana to destroy two lands is a good way to annoy players, especially in EDH
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