A Vallasvow is an oath of absolute vengeance sworn to the Vallas, the triad deity of the Skandian religion. To take one means not only to vow to kill a person, but every member of their family. This death vow is supposed to only be taken against treachery or murder, but this is ignored by Ragnak.
To take such a vow is considered madness by most Skandians, as it is unbreakable. Erak's opinion, ' "I told you, only a madman would swear to the Vallas!" ' (pg. 89, Flanagan, 2005) is similar to that of most Skandians. The person who breaks a Vallasvow will have every member of their own family taken by the Vallas instead.
A Vallasow was taken by Ragnak against King Duncan, which meant that in effect he swore to kill every member of the immediate royal family of Araluen. Many thought him to be not quite sane in his later years, though, and this was also done in the heat of the moment when Ragnak discovered that his son Gronel had died at the Battle of Thorntree, part of Morgarath's failed three-pronged attack against the forces of Araluen.