Aviarium: fantasy art of dragons, time-travellers and winged people

[Ierne]

i may have misunderstood physics, but I thought that a universe that was toroid-shaped in four-dimensional space would appear to be a sphere in three-dimensional space? Since it wouldn't be a regular toroid but a three-sphere, ie a projection of a sphere into 4-d space?

you're right, universes split very rarely. there are two main reasons why a universe would split:

1. the time-stream splits because the alternative consequences of an event are so significantly different that they can't be reconciled. this is usually a natural occurrence, but occasionally the result of time-travellers altering historical events.
this would only be a problem if the very process of creation itself were to be changed, resulting in an unstable universe. thankfully, this is outside of the capabilities of pretty much everyone ever.

2. the time-stream is split by a huge spike of raw time energy/magical energy. again, this is usually natural, but it can also be deliberately caused by insanely powerful time-engines or time mages.
this is always a problem. natural energy spikes can still severely damage a universe, and if someone is deliberately manipulating time at this scale then you have very, very big problems indeed. if someone is accidentally splitting universes then I have no words left to describe the scale of your problems.