True name

From Glen Cook Wiki

Every person in the world of the Black Company has a "real" name, usually the one they are assigned by their parents when they are born. These names are key points of vulnerability and control for anyone with sorcerous talent. Invoking a sorcerer's true name as part of a specific magical ritual can permanently strip him/her of all magical powers; using the name on a weapon (such as an arrow) will make that weapon deadly to the victim, even if he/she has taken steps to protect him/herself with spells. Casual use of a true name will not in and of itself be dangerous to a sorcerer; a specific ritual must be followed to use it against him, and the person conducting the ritual must have magical powers him/herself.

Most sorcerers (and for that matter most characters in this world) adopt nicknames or pseudonyms early on. In extreme cases, powerful sorcerers may erase their past by pretending to die and assuming a new identity, killing people who knew them back when they were young, or, in the case of the Dominator, destroying the entire province they came from.