I'm not really sure what else there is to do after the kill... I've explored both portals, you get limited time because you do die eventually and there is nothing there I could find to activate... I've tried jumping across to the light at the back of Atheon spawn platform, I can almost make it there on my Hunter by dancing across but its still not quite enough to get there. I've also climbed the glass pyramid above Atheon spawn point to as high as I could with no success. Perhaps it is more of a team thing though rather than a single person? There are the 2 pillars on either side of where Atheon stands when you kill him that may possibly need some action?