Black holes can merge
It is proven that black holes can merge with another black hole. It is theorized that a massive ripple effect or gravitational wave will happen if this occurs. This theory originated from Einstein’s theory of gravity and was approved in 2015, even winning a Nobel prize in physics.
The theory supports the idea of merges from large and small black holes or other stars, all found within the accretion disc. The effects will be massive and significant for any galaxy it happens in.