Is there a "Shadowdark" or similar in the Shadowfell like how there's a Feydark in the Feywild?

Question

The Material Plane (or Prime Material Plane) has an underground realm known as the Underdark.

There are planes in 5e that were introduced in 4e, the Feywild and the Shadowfell, that are reflections of the Material Plane.

In the Feywild, there is a realm called the Feydark, which is a reflection of the Underdark of the Material Plane.

Is there an equivalent realm within the Shadowfell that is a reflection of the Underdark? A "Shadowdark" or similar?

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I'm looking for lore from official 5e or 4e material, but I don't expect there to be any such lore from editions before 4e because the Feywild and Shadowfell didn't exist before then (although, since the Plane of Shadow from previous lore is considered to be the same as the Shadowfell, there might be something relevant from previous editions, but I'm expecting that most of the relevant information will come from 4e, since I couldn't find anything in 5e, which is the only edition I really know much about).

Answer 1

Yes

The 2010 4e book, Underdark describes both the Feydark and Shadowdark, where the Shadowdark is just what you are describing.

Additionally, the 2011 4e supplement, Shadowfell: Gloomwrought and Beyond, mentions the Shadowdark as well.

But the earliest that I think the word "Shadowdark" was mentioned in official material was actually something else: it's the area of the Upperdark (upper level of the Underdark) below Shadowdale, in the 3.5e published adventure, Shadowdale: The Scouring of the Land. This is not the "Shadowdark" you're looking for, because it's just "regular Underdark" under Shadowdale, but I wanted to point out that the word "Shadowdark" was previously used in this other sense.