D&D lore experts look here :)

[Paul Farquhar]

There are lots of pocket planes.

Or you could steel from Marvel comics and have a parallel prime material plane in which the gods of death where all killed and now everyone and everything is undead.

What's so unprecedented about Ras Nsi's power, anyway?

 

[Seramus]

Sounds exactly like the Negative Elemental Plane.

 

[ExploderWizard]

Sounds exactly like the Negative Elemental Plane.

DING! The negative material plane to be precise. The old school generic source of undead power.

 

[Zardnaar]

Negative energy plane last seen in 3.5 fits the OPs description.

 

[MarkB]

One of the D&D 4e adventure paths featured Death's Reach, essentially a prison plane within the Shadowfell, within which the gods had locked away primordials, demon lords and anything else they found too offensive to let free and too powerful to simply kill. Plenty of power to be had there, if you can tame it.

 

[MonsterEnvy]

Negative energy plane last seen in 3.5 fits the OPs description.

The Negative Energy Plane is shown right in the map of the planes in the 5e PHB.

Also out of curiosity Zapp do you want to just know Ras Nsi's source of power. Cause he has one.

Namely he is one of the barae the Chosen of Ubtao which granted him immortality in the form of being immune to aging and sickness. Along with a few other abilites. He also gained a unique power as a Chosen. Namely that he could animate the dead at will.

 

[CapnZapp]

Thank you 'vrybody

 

[Wiseblood]

Try the late Necropolis of the Ravenloft campaign (Azalins realm) Your should be able to reskin this.

I second this. IIRC the realms of Ravenloft were called the Demiplanes of Dread. Entities like Vecna and Strahd were called Darklords and within their finite demiplane they were boss. It is a weird place with strange like rules Power Checks and sanity. Lots of details and ideas worth mining can be found there.

 

[CapnZapp]

The Negative Energy Plane is shown right in the map of the planes in the 5e PHB.

Also out of curiosity Zapp do you want to just know Ras Nsi's source of power. Cause he has one.

Namely he is one of the barae the Chosen of Ubtao which granted him immortality in the form of being immune to aging and sickness. Along with a few other abilites. He also gained a unique power as a Chosen. Namely that he could animate the dead at will.

I'll probably use Negative Whatever Plane [emoji4] mainly because its not an inviting location to explore - the "stairs to nowhere" needs to be closed and that's it.

I don't think he got to keep his divine powers he got from Ubtao. In my mind necromancers utilize arcane power, from their own mind.

But ymmv

 

[AmerginLiath]

Another option is to use the infinite nature of the Abyss to have some entity quietly taken control of a “vacant” level and mess with the souls trafficking through. After all, Orcus can’t be the only major demon or demon lord who’s gotten a handle on controlling undead.