Musphelheim has two Nornir chests but only one of them can be opened using runes. If you are looking at the Nornir chest in the crucible area, this chest is unlocked by completing all 6 of the crucible challenges. Once they are all unlocked the chest will be unlocked.
If you are in the other area of Musphelheim, you will find a Nornir chest that is going to have you scratching your head a little. You will have to have made use of the Sigil arrows to chain explosions together by now but this one is rather difficult to get right due to the deep lava pit between the Nornir chest and the last of the three braziers.
You will find a brazier on the hill behind the chest which is easy to light using the chaos blades. You will find two more across the lava. The brazier on the left is easy enough to hit by using three sigil arrows.
Hitting the brazier on the right is the one that you are going to have difficulty with. There is no good way to describe it in words but I have included a video guide below that will show you how to his the third brazier in Musphelheim for the Nornir chest here.