When Unavaatu was purified, everyone assumed that the Dark Avatar was gone, at least until Vaatu was reborn. Twenty years after convergence, though, on the night of a full moon, Korra was attacked in her sleep and, stripped of her Bending. The next day Republic city was attacked by the Dark Avatar. The entire city's military was needed to drive him away.
Soon an army of benders and non-benders began attacking the various nations of the world, declaring that the world should no longer be divided. They fight to unite the world under one banner, the banner of the Dark Avatar.
With the armies of the world tied up fighting the United World Army, they can spare no one to track down the Dark Avatar and free Korra. If she could be returned to Republic City the Master Kya believes that she can use blood-bending to restore her powers.
A special task force must be raised to find her and bring her back. We are that task force.

I'd like to have someone for each element, and maybe a non-bender such as a chi blocker.