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The reason the X-men didn't tell the Inhumans beforehand is that the Beast, who had worked with the latter, didn't think it would do any good. The only option he presented was leaving the planet, and he talked about the retaliation if the mutants attacked the cloud. He even described how Black Bolt had killed his own son for the good of the Inhumans. They'd actually seen Medusa and Black Bolt apparently kill Cyclops who had tried to alter the cloud just so it wouldn't poison mutants. That led to its destruction later, but they didn't know that then. In #4, Iso admitted to Forge that she didn't know if she would have refrained from destroying his machine if she had known the truth. With all that, Medusa's line about terrigenesis not being worth the life of a single mutant seems false.
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