Dreaming Spectrum only drops on curated weapons. These can drop from encounters regardless of the challenges. I think each encounter has a specific weapon possible for a curated roll (Nation of Beasts from Morgeth, Tyranny of Heaven from Vault)
It's guaranteed to drop on a curated roll (you will know its curated as it will drop masterworked) and to get it you need to dismantle the drop.
English
-
Thank you!