it say in the guid firah and Kalia  that the CG it is found the caves of the ice raid. i cleared the 5 times qnd still no luck and what does the memorie fragment do?

forAelynthi CG how to get the legendary bow Molten Fury?

There are two spots in the ice raid where you get a card that allows you to enter a cave. There is a random chance to get the option to have time alone with Firah or Kalia in the cave, but it doesn't always appear.
The memory orbs can give you the CG scenes for the ice dragongirls, and Tiena and the Frozen Queen.
The legendary bow Molten Fury is one of the legendary weapons that you can get from the fire raid area.