To get the treasures, first you will have to gain access to Niflheim. To do this you will need the 4 Language Ciphers found in the purple chests with the mask floating in the center. See Trilingual for the locations and how to unlock Niflheim. Once in Niflheim, you will have to complete the side quest "Ivaldi's Workshop" in order to gain access to the central chamber, which is given to you by Sindri.

There are 5 treasure chests you will need to open and 3 realm tears to defeat. In order to open the chests and tears, you will need 68,500 Mist Echoes and 3 Anchors of Fog. You can farm the Mist Echoes by opening chests while you go through the dungeon and Anchors of Fog are round randomly in legendary chests. Every time you enter, the traps and enemies are different. Take care not to die as well, you will lose all you've collected during the run. So if you're worried, take a resurrection stone with you. If the timer runs out, then you will die as well.

You are also able to sell stuff for Mist Echoes, but only stuff you find in Niflheim will work. You can sell them at the Dwarven shops. The more chests you are able to open, the more echoes you will receive per chest during your run. I suggest crafting the Mist armor set when you are able to do so, as it will help with your survival time while in the mist, plus it is the best armor in the game. Once I had the set crafted, I never fell under half way on the time limit.