Forgotten Soul is the legendary-quality material for level 61-70 items and crafting recipes in Reaper of Souls. It is used in all legendary and set crafting above level 60 in Reaper of Souls, and is also required to enchant legendary or set items above level 60. Below level 61, the Fiery Brimstone material fills all these rolls, and that is the end game legendary material in Diablo 3.
Forgotten Souls are obtained chiefly by salvaging level 61-70 legendary or set items. It may also be dropped directly by level 61-70 Rift Guardians.
Forgotten Souls are account bound (BoA), and they can be stacked up to a maximum of 1000. They are the chief bottleneck for all end game crafting and legendary enchanting, and most players feel it is impossible to have too many, since any new legendary item find might might require dozens of enchants to roll the perfect affix.