Bolverk, meaning worker of evil or doer of evil deeds, is one of the sacred lukoi words and another name for Odin. To the lukoi, bolverk is a person (wolf) who meets out punishments that the Ulfric does not feel comfortable giving. Most packs do not use this title anymore, as most Ulfrics choose to do their own dirty work. Choosing a bolverk could even be seen as a sign of weakness.
The bolverk is allowed three helpers: Baugi, Suttung, and Guunlod. However, we have yet to see any of these helpers identified or even mentioned again after the initial introduction of bolverk as a concept.