Shadowfell Schism

The Raven Queen’s servants often seek out mighty allies. One of these is the black dragon Quetzallus, who resides in a deep cavern on the Shadowfell guarding a stream of souls flowing into the Raven Queen’s realm.

But a death knight named Raxikarthus, betrayed by the Raven Queen in life, now seeks to enslave this dragon for his own purposes. He madly believes that he can stand against the Raven Queen, and seeks allies of all stripes—even her own servants—to do so.

The death knight has found a pathway into the Shadowfell thought closed by mortals. Beneath a decayed keep, the undead dragonborn has reopened this portal and led his forces into the Shadowfell itself.
The PCs are called in to beard the death knight in his lair. They find it abandoned, but discover a strange passage in the lower reaches.

Pgs. 162-167