How to get the item? Besides fighting the weekly bosses, you can get a “Dream Solvent” from Zhongli’s Story Quest. Once you have access to the bosses outside of their quest, you can get Dream Solvent.
What does the item do? At the Alchemy Workbench, you can use the Dream Solvent to transform your boss items of the same category. This means, for example, that the “Tail Feather of the East Wind” can be changed to the Claw or Breath of the East Wind, but not to the other available boss items.