What happens when two black holes collide

The BBC has an article by Michael Marshall about a space spectacular:

Black holes are creepy enough on their own: dead stars that are so massive, not even light can escape their gravitational pull. The thought of two of them at once just seems too much.
Nevertheless, pairs of black holes do seem to be out there. If they get too close to each other, they could collide. The black regions are the event horizons of the black holes: anything inside these regions could never escape. The strange distortions to the background stars are caused by "gravitational lensing": the intense gravitational fields of the black holes bend the light rays coming from the stars, creating weird patterns.
The video was produced by the Simulating Extreme Spacetimes collaboration.

Rico says it'd be something to watch, but only from a safe distance., like a couple of parsecs...