There's Pluton, the former realm of the death god Nerull before his own death(ha!).
''Pluton, the Gray Waste
A dominion of dying willows, shriveled olive trees,
and black poplars, Pluton is forgotten by all but the
most learned of sages. This cheerless land was once
the domain of Nerull, a god of death who trapped
the spirits of countless mortals in his kingdom,
holding them in thrall. Some he punished with terrible
torments and some he forced to serve in his
grim legions, but most he kept as powerless shades,
aching for the release of true death. Nerull wanted
to reign as the king of the gods, and he sent curses
and plagues against the mortal world to swell the
numbers of the host of the dead.''
If not, you could go with the angle of the dead civilization, I think this subject might interest yuan-tis necromancers since its so close to their own reality:
''Once a mysterious mortal race of great knowledge
and enlightenment made its home in the dominion of
Shom, the white desert. Pilgrims journeyed to this plane in search of
truth and insight and to learn from the elders of Shom.
But the ancient race vanished long ago, leaving behind
its deserted temples, its buried mausoleums, and its
monuments of stone. Now Shom is a magnificent
wasteland, a desert of white sand where titanic statues
and empty cities silently guard their ancient riddles..''
And for extra edgy-ness, did you know that even the Shadowfell had its own Underdark?:
''Below the Shadowfell’s twilight landscape lies a nightmarish
reflection of the natural world’s Underdark.
Called the Shadowdark, this pitch-black expanse
features labyrinthine tunnels, vaulted chambers, and
subterranean seas. Past the lightless corridors, bottomless
abysses are cloaked in blackness and bearded with
ice. Oceans of necrotic seepage and darkland tunnels
are so malevolent that to walk them risks annihilation.
One constant in this endless expanse of darkness
is the cold. Descending into its depths plunges
travelers into icy stillness, cold enough for breath
to form and hang in the air. The deeper one travels,
the colder it becomes, until even those protected by
potent wards find the depths unbearable.
Like the Underdark, the Shadowdark is a refuge
for aberrant horrors such as creatures spawned or
touched by the Far Realm. Mind flayers, grells, and
other vile creatures hold vast sections of the Shadowfell’s
bowels, but its depths shelter even stranger
creatures. Ineffable horrors, grotesqueries born from
nightmare, roam the bowels of the underworld. Forgotten
kingdoms of undead are prevalent. Endless
tales are told about great catacombs infested with
zombies, dark cities overrun by wights, and fleshy
slicks of discarded body parts awakened into nearsentience
by powerful magic.''
