They are a part of Roshamuul Quest, needed for the mission that is also named Ancient Dreams.
I made a tutorial of the quest here and at 14:23 I'm showing how you get the Ancient Dreams.
You start off by going to NPC Morax
and starting the quest by saying "Hi Ancient Yes" and then to Roshamuul Mines here: 
Jump down into the mines and open this Box
to get a Dream Nebulizer
. 
To mine the Ancient Dreams
you now need to first use one of the Fog Lanterns
.
After you have used one of the Fog Lanterns you will be invisible and for 10 seconds have the ability to mine the Ancient Dreams. (Utana vid and Stealth Ring does NOT work as substitutes for this.)
Use the Dream Nebulizer on the Traces of an Ancient Dream
found here:
Cooldown on Fog Lantern is 2.5 minutes and Traces of an Ancient Dream is 5 minutes.
When mining the Ancient Dreams, you can also fail and get trash or spawn a Feversleep or Terrorsleep.
You can mine 10 Ancient Dreams from the Dream Nebulizer before it breaks.
There is more Traces of an Ancient Dreams and Fog Lanterns throughout the mines but I usually just Firebomb the first floor (-1) and farm there.
I hope this helped. Good luck!
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