Board Thread:Q & A/@comment-24749544-20170720090023/@comment-5896827-20170830015907

I think chaining them works so long as it is magical bonds for obvious reasons. It not only suppresses their powers, but I suspect it also severs their link to their other forms. If they can't use their powers, they can't spread their consciousness throughout nature like they usually do (as that is a supernatural power). So, the forms will outside of the bonds will disappear because they have nothing supporting them. They are essentially as powerless as mortals except they have immortality going for them.

This is what I think.