Forgotten Soul is a Legendary crafting material in Diablo III, obtained by using the Blacksmith to salvage non-crafted legendary weapons and armor, or set items.
A Rift Guardian may also give up one or two of these upon defeat for players in the level range of 61-70 (guaranteed to drop at least one on Torment VII or higher), and Odious Collectors may drop a few as well.
Each legendary item salvaged will yield 1 Forgotten Soul. An Ancient Item rewards 3, and a Primal Ancient Item rewards 15.
All Forgotten Souls are account bound.
Prior to patch 2.3, it was used in crafting legendary and set items of level 61-70. In current version of the game, it is only used for enchanting items, or Kanai's Cube recipes.