Soul of a Crestfallen Knight: Dark Souls 3 Consumable Item
Use it to gain 10,000 souls or sell it for 20,000 souls.
You can get this item by finding it near the Old King’s Antechamber or on the corpse below the staircase, where a Silver Knight is Kneeling.
The Unkindled can absorb the soul of a mighty Crestfallen Knight, as the Fire Keeper can use them to tap to, retrieving the strenght of this mighty fallen knight.
They say that when you kill a man, you take their soul, well this Order Knight took a lot with him, so the strenght of his own soul is considerable.