What is the bar on the hard mode icon?

I got this little bar on the bottom of my hard mode bar that is filling up as I kill guys... what does it mean?

It's an indicator of how many you've killed vs how many you've got left to kill. The fuller the bar, the fewer enemies left.

And once the bar is completely full, you have "Vanquished" the area. That means you killed everything that was there to kill... excluding pop-ups you didn't trigger.

What does "vanquishing" the area do for you?

Will help you max vanquishing title.

It also gives you exp and and gold depending on how many foes were in the area.