In the days before Sunless Skies was released, Failbetter released these Correspondence sigils. Thanks Lord_Norjam for the compilation!

The Correspondence
is a mysterious language of unknown provenance that only the finest scholars of the Neath can comprehend. There's quite a bit of rumor and hearsay about it, so that is listed below.

It is a well-known fact that Correspondence sigils adorn the spires of the Bazaar, as well as places around the Unterzee like the Avid Horizon. It's also known that trying to comprehend the sigils can lead to madness, and that badly written Correspondence (not having six sigils per sentence, or seven per name) can have harmful, even fatal, fire-related consequences. Specifically lilac fire.

What is the Correspondence?

They say it's a gate that opens in the stalactites behind Wolfstack Docks. They say you can see it in Mrs Plenty's mirrors. They say it's the only sure way to tell the weight of your soul.

They say it's a series of confidential negotiations between the Masters and a devil of some note. They say it comprises the billets-doux written by Jack-of-Smiles to the Traitor Empress. They say it's the letter the Pope wrote, the one without which Rome would have been the Fourth City. They say if you read it your eyes boil and your hair turns the white of old ice. They say it's written on slate in the blood of poisoned bats.

They say it's the key that opens Mr Stones' vaults. They say it's concealed in Mr Pages' library. They say it's the only way down here you can ever see starlight.

They say it's the language the bats speak. They say the Snuffer wrote it on the outside walls of New Newgate. They say the Topsy King learnt it, and that's why you can never understand a bloody word he says.

They say it's the letters that Helen wrote to Menelaus in the years of her imprisonment. They say it's the letters Raffles wrote about the Cat that never were published. They say it's the last accounts of the last days of the Third City, strung in beads on cord in a code no-one living understands...

They say it's the only map of all the Unterzee, scratched on the keystone of the Neath. They say it predicts every price change in the Bazaar for the next hundred years. They say it's a script that you cannot write and live. They say every piece of deep amber has a fragment of the Correspondence at its heart.

They say it's the map that connects every glimmer of moonish light to a star. They say it's the key that unlocks the secrets of bat-flights. They say it's a trap that someone found inscribed on a wall in the First City, and if you decode its complicated patterns you inevitably decide you're God, to the considerable detriment of your social life.

They say it's the mathematics of Hell. They say it's the geography of Time. They say it was invented wholesale by a honey-sipper sitting giggling in a cramped and filthy room on Hollow Street, and it's been driving gullible scholars insane ever since.

They say only the Brass Embassy knows.

 What is it really?
The language of the Judgements - the stars, like our Sun - and the lorn-flukes.
Discordance? [Spoilers for The Persona Engine ES]
It seems the Correspondence has an icy counterpart language, called the Discordance, the space between stars. We don't know much about it, however. Yet.