So I'm seeing a lot of people pointing to Argus being where we go to end the Legion, but that doesn't make sense. At this point it's probably just a smoldering hellhole full of demons, and killing the demons on that planet is pointless because you need to kill them in the nether to get rid of them for good.
I could see us fighting KJ or something considering it's an Eredar hotspot, but again it's pointless unless it's in the nether.