Four Forbidden Steps (concept)
The Four Forbidden Steps are four parts of the Thirteen Abyssal Steps to achieve Lichdom. It is unknown why exactly they are forbidden, or by whom. Many scholars believe that knowledge of them have simply been lost since the rise of the first reported Lich, Vilas Venslau, while others believe that knowledge of as many of the Steps as possible has been long repressed by the powers that be.
Discovering the Four Steps is generally agreed to be the first step towards achieving Lichdom, and speaking of them is generally considered extremely taboo if not outright forbidden in nearly all cultures and religions that know of Lichdom.