What happens when two black holes collide?

 

As a result of their collision, enormous amount of energy releases. The two black holes may merge into a larger one resulting emission of gravitational wave. Let's see an example:

First detect black hole merger is GW150914.Two black holes with masses 36 and 29 solar masses collided.The final black hole had a mass of ~62 solar masses.The missing 3 solar masses (about 5.4×10^47 joules) were radiated away as gravitational waves.This is more energy than all the stars in the observable universe emit as light in a brief moment.

A single supernova releases about 10^44 joules.A gamma-ray burst releases about 10^46 joules.A black hole merger like GW150914 released 10^47 joules—the largest energy release ever observed.