Ghost Of Diggen Thrune (1-2)

Diggen Thrune's ghost cannot leave the keep while Nashaxian still watches it. If Nashaxian is rendered unable to watch over him (such as if the PCs bind the devil to the keep), then Diggen can leave the keep. Still, his soul cannot pass on to the afterlife until he fulfills the terms of his contract.

Diggen Thrune can occupy a magical object. Diggen can't be targeted or harmed while occupying an object, but he can only exit the object with the assistance of a mortal in possession of the object.

Setting Diggen free from his contractual obligations with Nashaxian allows him to move on to the afterlife.

When a ghost is destroyed, it re-forms after within the location it's bound to, fully healed. A ghost can be permanently destroyed only if someone determines the reason for its existence and sets right whatever prevents the spirit from resting.

Diggen Thrune laments his fate, forcing each living creature within 30-foot area to attempt a DC 21 will save. On a failure, a creature becomes Frightened 2 (or Frightened 3 on a critical failure). On a success, a creature is temporarily immune to this ghost's frightful moan for 1 minute.