She marks the end of the current dungeon. There is simply no further floor behind her yet. ;)
If you're playing a 19.06.11 version from before I fixed the bug you might have the issue where you need to talk with Dea from the right for the movement to work while talking to her from below causes a freeze.
But to say for sure what issue you have, I'd need a bit more details about where you are and what happens. ;)