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?
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).