The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
Explain to me please but also wouldn’t that have to mean there was a black hole really nearby
It’s a reference to a book. But essentially yes you would manipulate the curvature of the universe to put something similar to a black hole around the solar system so that nothing could escape. A Black Domain, or a Light Tomb.