Just a thought, if an event happened well beyond the observable universe that caused entire galaxies to be destroyed radiating from a point source event, what would it look like from our perspective and how close could it get on our observable horizon while still being unable to reach us due to expansion of the universe?
Obviously, the ability to “observe” in this context is extremely relative to the time scale, and far longer than even our species is likely to remain in recognizable form. I want to conjure a mental picture an era where a substantial portion of the distant night sky is a marching black void, and no one is entirely sure if it will halt from expansion or end everything in a flash one day.
If the sphere of destruction is propagating at the speed of light, then any observable effect reaches you at the same time as the sphere itself. Either you don’t observe it because you’re far enough away to be safe, or you don’t observe it because you’re dead the instant it becomes observable.
Incidentally, you might be interested in looking up the idea of false vacuum decay - although if you tend to get anxious about end-of-the-world hypotheticals you might prefer to give it a miss.