My money's on this connecting to Hell somehow
From Fallen London Wiki (Staging)
Action edit is not allowed on comment pages.
"Apocryphon" has religious overtones, and while "caustic" usually refers to chemical burns or corrosive substances, I believe it originally meant "combustible" or "burning". So religion + fire = Hell?
The Advent Calendar text saying it "causes ink to run, and words to shift and change" makes me think of the Iron Republic, although I suppose they're more about new laws changing the way things are than about altering records of how things were.
One of the Marigold Knights was a historian exiled for trying to "create a comprehensive analysis of the revolution", contrary to Hell which "rewrites its doctrine regularly". It's not much of a leap from there to rewriting history ...