The Dictum Mortuum, or "code of the dead" (literally, "dead maxim"), is a set of laws laid down by Charon and enforced by the Hierarchy governing appropriate wraithly behavior vis-a-vis the Quick. The Dictum Mortuum strictly governs how Stygian wraiths can interact with the living. As a rule of thumb, they can't: communicating, using Arcanoi that allow them to breach the Shroud, or otherwise impacting the living is strictly forbidden except to the Deathlords and those wraiths they empower to do so on an ad hoc basis.
The reasons for the Code of the Dead are relatively complex; Charon himself was sympathetic to shroud-crossing and for many wraiths, achieving Transcendence is not possible without some interaction with the living. However, the practice of certain Arcanoi, like Puppetry, opened the door to incredible abuse. In addition, various wraiths crossed the Shroud to encourage suicides and deaths in order to increase their own portfolio and personal power.
The abuses that led to the code's creation also led to a certain wink-and-a-nod relationship with various Guilds. While the Guilds were officially broken in 1598, certain Guilds became criminal immediately following the pronouncement of the Code. The exact degree to which they were considered criminal was generally a function of the Guild and its past relationship with the Hierarchy: the Proctors were not heavily impacted by the code, given that their Shroud-crossings were generally non-invasive and usually harmless. Conversely, the Puppeteers were explicitly hunted for violating the Code and were generally considered the most extreme violators.