Hiraeth
"Hiraeth" is originally Welsh, and its meaning is inherently tied with Wales and Welsh culture. It is best defined as a mix of longing and nostalgia for (the Wales of) the past. The literal English translation is "longing", but the world itself captures a lot more than that.
In the earliest citations, where it was used in Welsh poetry, it was often used to imply 'grief or longing after the loss or death of someone'. It has also been translated as a sort of homesickness for a home that never was or a home to which you cannot return.
Fundamentally, hiraeth is a feeling of sadness and longing; it is a deep-set yearning; it is the desire of that which cannot be and has never been; or only was once, and will never be again.
So what do you do when an entire world yearns for itself? For when a world is left in fragments, broken, trying to piece itself together when it was never supposed to exist in the first place?
This is the fundamental concept behind Hiraeth. It is a quest; it is a half-remembered lullaby; it is the broken world taking what it can from elsewhere. And now, people who had no reason to ever meet must learn to, somehow, get along in a world that fuels paranoia and encourages distaste, even hate, if they ever want to have a chance to go home.