The end of the universe itself?

Nonono, you don't get my point about density. It's possible to have a microscopic black hole, provided the mass is dense enough. I forget the limit this happens at, but it can happen. It's just it'll last for a fraction of a second before the repulsive forces make it vanish. (Because the repulsive forces >>> gravitational forces)

EDIT: Basically, it's where if light gets close enough to the black hole, it'll get sucked in.

Sounds good to me. I almost posted something about an atomic black hole, but then decided to keep things more simple, b/c there was no way this black hole was going to 'swallow the earth'.