Hello,
I am here because I want to suggest implementing the Ruathlek for ESF: Illusionists and Illusion clerics.
It is called Ruathlek and it was a language developed by Leira (deity of Illusion and deception) for illusionists.
The books state that the language is known by Illusionists broadly. So any illusionist who has learned the secrets of the school will most likely eventually learn the Ruathlek language. On top of that, Waterdeep (a city that Guldorand is formally associated with), has the largest known library of books written in this magical script/language.
Therefore, it makes 100% lore sense for illusionist spellcasters to know this 'secret' language.
Think of it like the Thieves Cant of Illusion spellcasters. That's literally what it is.
I think it'd be a super cool 'secret' language to implement for some cool intrigue rp for illusion characters.