Aklo
"I had to learn Aklo a bit at a time. It took me a while to realise that was the cause of some of the migraines."Cassiopeia Stargazer-Greenskin doesn't allow a few migranes to get in the way of her research.Associated with precursor deities. The secondary ancient language (after Ancient), but is moreso associated with the parts of ancient palimpsest culture that became forbidden or discarded. Ancient religious writings in Aklo (about gods that aren't the Eight). (Old Gods <-- Panopticon and Whirling-In-Rags and Worgoth and He-That-Walks-Beyond-The-Wall [bits that aren't in Necril are written in Aklo)
Part of the reason Aklo is illegal is that it’s a language that doesn’t tend to interact well with material life. Most languages were created and designed by living things, they evolved from living creatures trying to communicate with each other. So their construction and the way you need to think to make sense of the language is the way that living things perceive the world anyway. And then there are some languages that are harder to learn because they are not designed by creatures that think and perceived the way living things do. Elemental languages are tricky to learn, but elementals are close enough to material life… they aren’t damaging to try and read for any length of time. Some of the explicitly illegal languages are unsafe to read for any length of time, because of how you have to contort the mind.