When you start the Lumina quest you will get an item that has you tracking down another quest step. This item will give you a different location to go to and will change periodically. One of the locations that it may tell you to go to is The Mists on Nessus. While this can be a confusing area with multiple levels and plenty of Vex, you can get there very quickly using this guide.
To get the Mists treasure and continue the Lumina Quest you are going to want to head to the Artifact’s Edge Landing Zone on Nessus. You’ll need to jump down from the tall tower and then head into the area that is pretty much smack dab in the middle of The Tangle. In The Tangle you’ll find a glowing orange entrance that takes you into the Mists.
You’ll head through this cavernous area and then tucked away in the corner is the treasure that will allow you to continue the Lumina Quest.
You can use the map above to pinpoint the exact location, but just remember that this area is entirely under ground.