Slipstream Serpent 

Creature — Serpent
$0.04 #29362 0.12 20.0 years
This creature can't attack unless defending player controls an Island.
When you control no Islands, sacrifice this creature.
Morph 
(You may cast this card face down as a 2/2 creature for
. Turn it face up any time for its morph cost.)
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Cards similar to Slipstream Serpent 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.
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"When you control no Islands, sacrifice this creature."
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"When you control no Islands, sacrifice this creature."
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"This creature can't attack unless defending player controls an Island."
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$1.97↑#304740.1227.5 years↑
"This creature can't attack unless defending player controls an Island."
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"This creature can't attack unless defending player controls an Island."
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"This creature can't attack unless defending player controls an Island."
Hammerhead Shark96%
$0.17↑#253920.00↓28.6 years↑
"This creature can't attack unless defending player controls an Island."
Deep-Sea Serpent96%
$0.39↑#306500.31↑29.5 years↑
"This creature can't attack unless defending player controls an Island."
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$1.07↑#300620.00↓28.3 years↑
"This creature can't attack unless defending player controls an Island."
Zhou Yu, Chief Commander96%
$150.00↑#297150.26↑27.5 years↑
"this card can't attack unless defending player controls an Island."
Island Fish Jasconius96%
$0.32↑#269030.00↓32.8 years↑
"When you control no Islands, sacrifice this creature."
+1 more matching line
Merchant Ship96%
$42.56↑#287710.1232.8 years↑
"When you control no Islands, sacrifice this creature."
+1 more matching line
Ethereal Whiskergill96%
$0.25↑#310620.00↓19.0 years↓
"This creature can't attack unless defending player controls an Island."
Giant Shark96%
$0.31↑#260070.00↓32.2 years↑
"When you control no Islands, sacrifice this creature."
+1 more matching line
Slipstream Eel96%
$0.06↑#264140.0524.0 years↑
"This creature can't attack unless defending player controls an Island."
Kukemssa Serpent94%
$0.11↑#289480.00↓30.0 years↑
"When you control no Islands, sacrifice this creature."
+1 more matching line