Oh my god. These spoilers are so huge I have to hide them.
During Aang's time, there was this evil mob boss called Yakkone, and he was a blood bender who could do it at will. Korra kept getting visions of him. I thought they were implying that this is how the Avatar is killed or how team avatar is defeated, but no. Aang goes into the Avatar state And takes away Yakone's bending.
Here's the kicker. That prissy tarrlock is actually Yakone's son! And he trapped Korra! Dude is insane, what exactly what his end game here.