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When you control no Islands, sacrifice this creature.
You may choose not to untap this creature during your untap step.

: Gain control of target creature whose controller controls an Island for as long as you control this creature and this creature remains tapped.
If a creature’s controller doesn’t control an Island, that creature is an illegal target. Whether that player controls an Island is checked both when Seasinger’s ability is activated and when it resolves. However, targeting restrictions aren’t checked continually while a spell or ability is on the stack.