Bonecasting
Throughout the council territories, there are those who wield magic outside of the law. There can be many paths to power for these apostate mages, whether born with the gift of the arcane, forming pacts with powerful entities or secretly studying forbidden spells. They are often able to blend in with the mundane folk of the land, moving unseen, for if they are ever revealed they will be hunted and killed by an order of witch hunters called the Justicars.
Wielding magic without proper training is rightly outlawed as it is fraught with peril and can attract the attention of hungry demons, abominations or worse. Nevertheless, there are always those who seek access to the power of the arcane without the restrictions of the Spires and the Justicars. They operate in secret, in the shadows and must communicate without drawing attention less those who catch them try to claim the Apostate Bounty.
Bonecasting is a form of subtle divination magic that manipulates the traces of mana that echo in the remains of living things, those fluent in its practice can imbue bones with whispers and memories and leave them for others to find. normally these bones are collected from rodents or small prey animals but the larger the message the larger the bone required and it is not unheard of for humans to be killed just for their bones, for on such a canvas a volume of information could be stored. Typically Apostates will have appointed dead drops for these bones and use them as warning or messages between individuals or cabals of aligned mages.