The Four Great Souls are a Dark Souls 2 reference of the original Dark Souls boss souls. See Old Souls on the Dark Souls 2 Wiki. They belong to the Four Old Ones, which are all:
- Old Paledrake Soul
Remnants of the soul of Seath the Scaleless, found by defeating The Duke's Dear Freja (Brightstone Cove Tseldora).
- Old Witch Soul
Remnants of the soul of The Witch of Izalith, found by defeating The Lost Sinner (Sinner’s Rise).
- Old King Soul
Remnants of the soul of either The Four Kings or Gwyn, Lord of Cinder, found by defeating The Old Iron King (Iron Keep).
- Old Dead One Soul
Remnants of the soul of Gravelord Nito, found by defeating The Rotten (Black Gulch).
Collecting these 4 souls will allow progression through Winter's Keep into Drangleic Castle. The final area through this route is Undead Crypt where the boss is Velstadt, guard of King Vendrick. Near to Vendrick is the King's Ring, needed to gain access to Aldia's Keep, Throne of Want and Dream of the Giant.