two things One you havent't killed atheon so whatever you are trying to access isn't going to be accessible until you kill him. Two when you beat atheon you have a limited amount of time to explore before the reward screen pops up. Edit: i forgot to mention you can open the portals and if someone goes in you get times vengence. the person in there will be marked and then die a few secounds after their screen goes dark. when that person dies if someone else goes in they will die instantly. Thats all i know right now.