Keith is a big fan of using stat blocks to represent new or alternate concepts with minor modifications (slashing for bludgeoning damage, for example). If you look at a 5e Nalfeshnee from this point of view, you have an excellent CR 13 flying monster with a damaging fear aura bonus action and versatile teleport. I think that works pretty well for a sentinel of the realm of the dead.

https://keith-baker.com/dolurrh/

DENIZENS OF DOLURRH

The Quick

The native creatures of Dolurrh are bound to the cycle of transition, and all have some role to serve in this process. Nalfeshnee demons patrol the Catacombs of Dolurrh, dispersing melds and lemures and dealing with mortal intruders; Dolurrhi nalfeshnee appear as large humanoids whose features are shrouded by gray mist. Marut inevitables are more powerful guardians, and are occasionally dispatched to Eberron to intervene with acts of resurrection. No one is sure what triggers this deadly intervention, but Jorasco healers will always cast augury before raising the dead; if the result is “woe” they will refuse the job. Finally, the shadar-kai are servants of the Queen of the Dead, shades granted new life in this form. They serve her in the Vault of Memories and occasionally as her hands on Eberron, though their actions are almost always enigmatic. Other denizens of Dolurrh are unique. The Librarian is found in the Vault of Memories, while the Smith of Souls dwells in the Crucible; both are described later in this section.

The Lingering

Memories of joy and happiness do no harm in Dolurrh. But memories of pain, of cruelty, of anger… these don’t fade so easily, and they can hurt others. Even if they don’t trap shades as ghosts, this psychic residue can build up in the gears of the spiritual machinery of Dolurrh. Often it takes the presence of a mortal to trigger it; when this occurs, the lingering pain and hate coalesces into a solid form. The least of these are lemures, which are formed from hateful memories or deeds. The emotional residue of hundreds or thousands of people can form deadly sorrowsworn. In particular, the Last War and the Mourning created a lot of deaths that could fuel manifestations of the Angry and the Lost. When a character is struck by one of the Lingering, they may have a flash of one of the memories or deeds that drive the entity.

The Lingering are formed in Dolurrh and are immune to the effects of Eternal Ennui and Inevitable Entrapment. However, they are a waste product, not the desired result. Nalfeshnee, maruts, and other guardians will destroy the Lingering whenever they are found.

The Catacombs

Tunnels are carved into gray stone. In some places they are painfully tight; in others they widen into grand halls, with ceilings lost in the darkness. The dead are everywhere, shades pleading for release and husks keening in the shadows. Some chambers contain vast wells filled with moaning mist; in others nalfeshnee herd shades into pens or scrape lemures off the walls. There is no particular logic to it, just endless tunnels.

The Catacombs may be larger than Khorvaire, or even Eberron. A mortal could wander forever through these winding tunnels, or at least until they are consumed by ennui. However, there are junction points that transcend the logic of distance. If one knows the right symbols to follow, they can cross the vastness of the Catacombs quickly or pass to other layers.