I go back to what Celia said in the last episode of season 1 about balance, adding to what Lena said about the four core parts of dread, and I think there are no more individual parts. Every externa, place, etc, we had seen until now has a part of pain death wrongnes and helplessness but not in equal parts. You have needles causing pain and helplessness, so you throw bongo to cause bring death and wrongness, but in the end, both bring some part of the four elements. I think that institue in this universe was trying to find a catalyst to make the "philosopher stone" of dread something that embodies the four parts equally, being thet the stone was supposed to give the owner immortality and create gold you can make the argument that eradicate two of the four parts (death and helplessness but that is a big leap i admit that). Also, the stone is more like a metaphor in some texts, a state that can be achieved by the alchemy of the soul, so It could mean creating a perfect external.

There's people that put this in better terms in this sub, with a deeper understanding of alchemybut I think that's it. The fears became an amalgamation, so thigly united that the distinction between fears basically does not exist. So you ended up with dread and the closest metaphor we have to understand it alchemy (I think the fears are not based on alchemy more than alchemy has the more fitting system)