If you look at each of the 3 dungeon pages, each one will show the chest that this comes from! (There are icons of each of the drops on the page.) It looks like this item comes from the final chest in each of the dungeons.
Copperbell - Last Boss (Gyges the Great)
Sastasha - Last Boss (Denn the Orcatoothed)
Tam-Tara - Last Boss (Galvanth the Dominator)