Do you know this Justice League (Unlimited)/Batman Beyond episode?

All that I remember is that it took place in an alternate universe of sorts, and I know Batman was involved, but I can't remember if it was a JL or JLU or Batman Beyond (crossover) episode. There was a little girl who was causing a lot of bad stuff to happen (thus the alternate universe) and only Batman could reach her through this fortress or something that she had made. And I remember that she died at the ending and he stayed with her until that happened. Can anybody tell me the name of this episode and/or a link to it?