To clarify, think of that energy as consecrated ground in DnD except in reverse. The Vy'lar can only move in area's it covers, not area's it doesn't. See Fenris is just using a puppet, so only a small part of him is experiencing the Outside. For their true forms to even be able to exist there, I figure a little piece of home is needed.

Yes this energy can be blocked and even reverted.