Berserker introduces the "Third Force" into the In Nomine Satanis / Magna Veritas universe. The Third Force label designates occult forces independant of the forces of Good or Evil, such as beings from pre-christian pantheons. This sourcebook focuses on Vikings and Sorcerers.
Vikings are warriors who believe they have been recalled from Valhalla to take part in Ragnarok, the final battle between the Norse Gods and the Giants. They believe Demons to be the Giants, and Demons happily oblige (after all, viking warriors are less resilient than angels, and good target practice). The first chapters of the book contain everything needed to play a viking character, including character creation, powers, description of the pantheon, viking psychology and honor code...)
Sorcerers are human magicians, they seek knowledge and power by summoning angels or demons and attempting to bind them. Sorcerers are meant to be NPCs, but the rules cover everything needed to play one, including summon rules and some other spells.
The book ends with two scenarios: "Pour qui sonne le gland" is set during the battle for Berlin in 1945, the characters' goal is to eliminate Adolf Hitler. It can be played by angel, demon or viking player characters. "Qui veut la bite de Roger Rapeau" has the players (angels or demons) investigate a series of strange murders.