Description
This spectral weapon, often employed by the wights and the undead, has been harnessed and bound to the realm of mortals and is now possible to wield by human hands. The weapon drains stamina from anyone its blade touches.
Notes
This weapon is much lighter than almost any other. It also has no durability, making it excellent for farming.
Due to the nature of this weapon, it cannot be upgraded.
Source
The Ghast Sword can be found in 
the Unnamed City, by looting boss creatures.
Repair
This item can be repaired with a Legendary Weapon Repair Kit.