Below, you can find the full FFXIV: Heavensward item info for the Soul of the Dark Knight item. This item is a Soul Crystal and is usually a crafting material or the product of a craft. Sometimes it can be dropped from a monster or found in a chest.

Soul of the Dark Knight - Amulets - Items

Soul of the Dark Knight

Soul Crystal

DRK

Level: 30
Item Level: 30

NPC: FFXIV Gil 0

Upon the surface of this multi-aspected crystal are carved the myriad deeds of dark knights from eras past.