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Languages: Elven and Common
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The gods built the Fringe as a mirror, and the Shadowfell side is the most literal, and tragic, reflection.
The Inhabitants (The "Natives")
Unlike the Feywild, the Shadowfell-Fringe was not a stable, external empire. It was a dark, volatile land, seeded in Year 0 with the peoples who thrive in such gloom: the Shadar-Kai (elves who embrace the shadow) and the Gloom Dwarfs (dour, pragmatic, and resilient).
The Great Power (The "Secret God")
The "greater power" of the Shadowfell is not a single, unified king. It is the mysterious, fleeting, and terrifying Raven Queen, the mistress of fate, death, and memory. As you've speculated, for the purposes of this world, she is the "divine mask" or true, hidden identity of Suel, the Goddess of Death, Secrets, and the "Psychopomp."
The "Native" Kingdoms (The "Emotion States")
Suel's (the Raven Queen's) domain does not have "vassals"; it has manifestations. The political states of the Shadowfell-Fringe are small, isolated, and "fleeting," as their very existence is powered by the "emotional bleed" from the mirrored Material Plane.
When the "Great Anarchy" began in the Forsaken Demesne (Y79), the 45-year period of war, trauma, and "screaming in their sleep" became a massive source of psychic fuel. This "bleed" empowered the "native" inhabitants of the Shadowfell-Fringe, who then formed kingdoms based on these very emotions:
The Queendom of Anxiety: This kingdom is ruled by Alya Quintessa, the Shadar-Kai who (as you noted) was likely created for this role. She is the literal "Queen of Anxiety." Her kingdom is not a "place" so much as a political state fueled by the collective, baseline fear, paranoia, and stress "leaking" from the 87% "Wilds" of the Demesne. Her subjects are Shadar-Kai and other "spirits collected from the dead" who thrive on this specific emotional resonance.
The Land of Depression: This kingdom is ruled by the Gloom Dwarfs. It is a place of absolute, crushing ennui and hopelessness, a direct reflection of the 45 years of trauma and loss suffered by the Aasimar veterans (like Narcis's clan) in the mirror plane. The Gloom Dwarfs did not create this state; they simply "adapted" to it, building a dour, joyless, and rigid society that perfectly mirrors the endless, "screaming in their sleep" trauma of the Demesne's clans.