In multi-player games you can choose a different opposing player each time it is used.
As you activate the ability, you choose an opponent, then that opponent chooses a creature for the ability to target. It doesn't have to be a creature that opponent controls. If there are no creatures that can be targeted with the ability, it can't be activated.
As the ability resolves, you choose a damage source.
The next time damage from the chosen source would be dealt to you this turn, that damage is dealt to the targeted creature instead. Whether it's still a legal target is not checked at this time. However, if it's not able to be dealt damage because it's neither a creature nor a planeswalker, or it's no longer on the battlefield, then the damage is not redirected and is dealt to you as normal.