Mirror Realm is what Mordret called the reflected place inside the in the mirror's surface or any reflective surface.[1]
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Description
The space only contained the part of the world that reflected in the mirror's surface or any reflective surface, and a small area around it.[1] The entire place had a limit and flipped from left to right.[1] The world inside the reflection was inverted.[2]
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Mordret's Realm
There was a vast and somber expanse around them. The grey sky was covered by stormy clouds. The surface beneath their feet was shrouded in white mist. The whole space was suffused with soft light… and there, high above, a single radiant sphere was shining like a sun, its pale rays falling through the veil of storm clouds.
It seemed as though vague symbols were carved into the surface of the sun, but Sunny could not see them clearly.
This place… resembled a Soul Sea.
— Reflection of Mordret's Soul Sea, Chapter 1499 Prince's Old Clothes
Function
It was like a pocket dimension where people can be pulled inside.[1][3] It was unknown if one can escape being trapped inside or if it can connect two reflections at the same time, bridging two physical places.[3]
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Trivia
It is considered an extension of Mordret's Ascended Ability.[1]
Inside the Mordret's realm, there were motionless figures of people and Nightmare Creatures standing in a row. They were standing in the mist with empty expressions and hollow eyes. They were his vessels.[3]