A word or phrase to describe events that happen alongside major historical events

I am looking for a word or phrase that could be used to describe events that happen in at the same time as major historical events. Examples include the darkness that occurred during the crucifixion of Jesus Christ, and the comet that appeared in the sky after the assassination of Julius Caesar. The best I could come up with is omen, but I don't think that is correct.

EDIT: After reading a comment, I see that I failed to describe that I am looking for a word in the context of these events being perceived to be related, as if the comet was some sort of a way to show the importance of what had just happened.

Solution 1:

They are coincident events.

From MW dictionary:

Simple Definition of coincident : happening at the same time

the hard economic times and the coincident increase in crime were a double strain on the city's social services

Common synonyms are simultaneous and synchronous. Also, contemporary, though a synonym, is more of an approximation.

Solution 2:

To me omen precedes the event associated with it.

When something happens alongside or immediately following an important event otherwise would be a symbol of the import.

Additionally, the event following or extending the duration could also mark, validate, endorse, authenticate, manifest, and indicate.