Oh yeah, them. Most of this is second hand knowledge, but sounds like they were some kind of crazy commando squad of nutbars. Led by this evil airbender, Zaheer. Wanted to get rid of all the world leaders, turn the world into some sort of happy free-for-all with no governments! Like that would work out right, buddy?

[Varrick gives a snort.]

They tried to off Avatar Korra, too, but they managed to kick 'em where the sun don't shine.