Thanks to those of you who have offered to help. The complete sentence is as follows. Widziałem w przyszłości różne cuda, ale cóż, kiedy i tam, gdzie mieszkają tylko czarownicy, nie można żyć wiecznie.
You should have started with quoting this sentence at the very beginning! Now I know what your problem is re: understanding. In this case it means.
I saw various miracles in the future, but what of it, when also there, where only wizards live, one cannot live forever.
This is a piece of literary language and you can only translate it properly if context is also given. Did that translation help?