Hung drawn and quartered

Hanging, drawing, and quartering was the medieval penalty for high treason against the English crown, though it was not formally established until 1351 (considerably later than a typical Ars Magica saga).

Authors of Ars Magica products sign a legally binding Non-Disclosure Agreement (NDA) saying they will not discuss contents of the product before it's published, nor details of the development process. According to legend, any author who breaks this contract will suffer a punishment too ghastly to even contemplate.

Realistically, what this probably means is that said author(s) would be barred from further writing for the line.

Playtesters also sign NDA's and have therefore sworn weighty oaths of silence, on pain of similar gruesome punishment (not being allowed to playtest any more).