Enochian

Enochian was first created by John Dee and Edward Kelly who called it the "language of the angels." Which is partially true, but the arcanists developed a separate version of it that was more suited to their needs. To this day, many arcane practitioners use the language to share messages between themselves in order to prevent others from deciphering it.
Learning Enochian is one of the important aspects of having an actual mentor who introduces you to Arcana, rather than exploring the concept on your own. Even if you set out to do your own research later, you have access to a much larger collection of knowledge. Sometimes important texts are even available at your local library without them understanding the power within their volumes.
Some throughout the times have claimed that this language is in fact divine in order to get various churches to protect the texts. But there are also many who think of them as occult devil-worship. Today, conservative areas might not have anything in Enochian available anywhere public.

Writing System

by Wikimedia Foundation

Phonology

Due to its origins, much phonology and phonetics are similar to Old English, but with a few twists. Vowels tend to be more open, while few consonants are further back than alveolar.

Morphology

The morphology, as well as the syntax of Enochian, can be extremely irregular. In fact, this is one way in which messages are hidden from unintended readers. Some outsiders have even misconstrued this as the language being nonsense.
There are several parts of writing Enochian that fulfil no purpose except being placeholders to cause confusion. These parts are then taken out if the message is supposed to be used for a ritual. This is why rituals with complex instructions still end up with sigils with only a few symbols representing the whole.