Featherless Creature

Ralsei first calls them "An alternate view of reality through the lens of shadow." Now, I don't know if I'm alone in this, but that doesn't really scream "illusion" to me. First of all, alternate implies some level of equivalence, but that's pretty weak evidence

What especially leaps out at me, is the "alternate view" part. It reminds me of–bare with me–reinterpret casts in programming.

The non-technical summary is that data on a computer is all of the same type; binary. Binary data can, however be interpreted in a lot of different ways. A signed floating point number and an unsigned integer could both be 64 bits, but what they mean is different.

Some data types can have multiple valid interpretations. For example, an ascii character is also an 8-bit (technically 7 depending on implementation but shhh) number.

So, basically, Dark Worlds and their Light World counterparts both share the same underlying data, interpreted in a different way. So, when Berdly hurts his arm, that's because taking damage in the Dark World changed his information in the same way that getting his arm paralysed would in the Light World.

TLDR: Dark World is a different way of seeing the world, meaning that actions in the dark world do happen, but don't correspond to the exact same things.

The one problem with this theory is that... Darkness seems to correspond to things getting more indistinct, which doesn't quite reconcile properly with the idea that Dark Worlds are reading the same data.

My personal headcanon is that Dark Worlds make things so indistinct that anything can happen, but like. What's happening still gets reflected in the light world.

Imagine it like the Darkness stretching out the data of the light world, and filling in the gaps. The Light World data still exists and can be changed by stuff happening in the dark worlds.