Harbinger of the Seas 
Creature — Merfolk Wizard
$1.56 #3774 1.41 2.2 years
Nonbasic lands are Islands.
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Cards similar to Harbinger of the Seas expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Magus of the Moon98%
$3.97↑#32851.75↑19.3 years↑
"Nonbasic lands are Mountains."
Blood Moon98%
$7.32↑#1485↑2.13↑32.1 years↑
"Nonbasic lands are Mountains."
Conversion86%
$0.24↓#26556↓1.18↓33.1 years↑
"All Mountains are Plains."
Zhao, the Moon Slayer85%
$0.32↓#10209↓0.00↓0.8 years↓
"Nonbasic lands enter tapped."
Glaciers84%
$0.54↓#29115↓0.69↓31.3 years↑
"All Mountains are Plains."
Pale Moon77%
$0.46↓#27758↓0.75↓26.6 years↑
"Until end of turn, if a player taps a nonbasic land for mana, it produces colorless mana instead of any other type."
Illusionary Terrain76%
$0.33↓#25377↓0.91↓31.3 years↑
"Basic lands of the first chosen type are the second chosen type."
Yavimaya, Cradle of Growth76%
$15.42↑#77↑0.29↓5.3 years↑
"Each land is a Forest in addition to its other land types."
Urborg, Tomb of Yawgmoth76%
$53.04↑#73↑0.49↓19.6 years↑
"Each land is a Swamp in addition to its other land types."
Blanket of Night74%
$1.11↓#16594↓0.33↓29.6 years↑
"Each land is a Swamp in addition to its other land types."
Quicksilver Fountain74%
$3.56↑#13382↓0.57↓22.9 years↑
"At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land they control of their choice. That land is an Island for as long as it has a flood counter on it."
Prismatic Omen73%
$3.21↑#35110.26↓18.3 years↑
"Lands you control are every basic land type in addition to their other types."
Blood Sun73%
$0.40↓#15783↓1.12↓8.6 years↑
"All lands lose all abilities except mana abilities."
Alpine Moon73%
$0.25↓#24377↓0.43↓8.1 years↑
"As this enchantment enters, choose a nonbasic land card name."
Back to Basics72%
$3.56↑#5287↓2.23↑27.9 years↑
"Nonbasic lands don't untap during their controllers' untap steps."
Winter Moon72%
$0.53↓#2832↑2.08↑2.2 years
"Players can't untap more than one nonbasic land during their untap steps."
Stormtide Leviathan72%
$0.51↓#2864↑0.94↓16.1 years↑
"All lands are Islands in addition to their other types."
Living Plane72%
$210.29↑#17827↓0.97↓32.3 years↑
"All lands are 1/1 creatures that are still lands."
Nature's Revolt72%
$5.30↑#13033↓0.80↓28.9 years↑
"All lands are 2/2 creatures that are still lands."
Ruination72%
$0.55↓#8278↓1.99↑28.5 years↑
"Destroy all nonbasic lands."