This story is a sequel to Dark as Chitin

Proceeding the events of Dark as Chitin, our protagonists finally have time to relax and settle into their roles within Equestria, or so it seemed. Chimera and Luster Dawn are faced with a new challenge, guardianship of a, particularly naughty filly. Chrysalis, on the other hoof, is having trouble re-entering society and conforming to the norms of pony-kind. Meanwhile, Soul's adjusting to his role as The Lord of...Nothing.

As everypony carries on with their lives, a threat looms just outside the edges of perception, possibly threatening all that ever was, is, and will be.

Our protagonist, a changeling created before the renaissance of its species, wakes up trapped within a cocoon-like egg of sorts. Upon breaking free, he finds that he is underground, alone, and in the dark. He has little to no memories of who he is nor how he got here, yet he feels like he should.

To top it all off, there's a disembodied voice in his head claiming to be the queen of changelings and she has very big plans for him. He can't help but feel like he has his work cut out for him. What a doting mother...

Sequel: https://www.fimfiction.net/story/497868/dark-as-chitin-soul-theory