Hi huckpinn,
Whoah, I didn't know that could happen. A messenger should have come to you letting you know there was a problem. Did you move on to chapter 4? If you did you can't go back to the mainland until you finish chapter 6, so if you did, that could account for why you lost it. If Nalia is still with you, you could try going back to the keep and try taking it back through warfare. I don't know if you can get it back or not but it may be worth a try if you defeat all of them.
GL
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