You'll have to kickstart the backup generators in Limbo in order to reach Mundus behind the vault door. There are four generators and paths, with their own share of the game's last few remaining collectibles
. Let's start by turning around looking behind where you came from to find a vitality star. Next, go to the southeastern path first.
Follow the path until you reach the generator. Before kickstarting it, look up to the left to find a lost soul.
Use demon pull to start the generator then kill off the enemies who will attack you. Return to the main area then take the northeastern path this time. Once you encounter a chasm, look down and you should see another path leading to the left. Jump down and follow it until you find an Argent Key.
After collecting the key, jump to the pit and head to the left to find the elevator. Kickstart it and defeat the enemies trapping you in the area. Return again to the main area then take the southwest path this time.
Continue to The End
of the path. Before jumping below, stop at the edge and look to the right to find another lost soul. Double-jump before angel boosting to reach it.
Make your way to the last generator, activate it and defeat the enemies who will ambush you. Finally, make your way to the last generator. Here, the last door and lost soul will be right in plain view so it's impossible to miss them. (Argent – Colossal Triumph)
Argent Door and Final Lost Soul, Demon's Den
Continue to the generator and activate it. Defeat the enemies and backtrack to the main hall. Next, you have to line up the icons on the floor by stomping them. Line up all four to open the vault and complete the mission