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We have a Siren that used it's Siren call against Crevere while it was in her zone. Naiya is in an adjacent zone. Now Crevere activates. Since in the beginning of it's activation he cannot move closer to the siren, he don't have to do so. Let say he moved to Naiya's zone. He isn't hindered by anything (in this situation there were no shallow sea in either zone), so now he can move closer to the Siren.
Must Crevere now return to the Siren's zone because now he can, or could Crevere now attack Naiya since he couldn't move toward the siren at the start of the turn so he no longer required to do so?
[the same question could be asked about a flying creature as well, since they are not hindered in this case as well, but the answer should be the same.]