A massive star obiting a massive black hole dies and creates another (massive) black hole. If these were close enough together, what would happen?
In the centers of galaxies are black holes, enjoy. But you really should ask on an astronomy forum. If you're boring like me, perhaps give a listen to some lectures. https://www.youtube.com/watch?v=42cqhjmt4vs
A black hole collision has never been observed. In theory, they will merge together, creating massive, rippling waves of gravitational energy. It would be devastating for any nearby bodies.
"The human mind is the most perplexing and illogical conjunction of insanity in the known universe. Leave black holes be human." - Spock