The Lethal Oath is the second book in The Viking Series, and continues the late-Viking Age saga of warrior Sigve the Awful.
Sigve is now the youngest chieftain in King Godred's realm. During the inaugural feast, he swears a sacred oath on his blade, the very sword responsible for his reputation. The sword is bound by the sacred spell of the slayer rune, and breaking the oath will have dire consequenses. Sigve soon finds himself in conflict with both his mother and his captain-of-arms, overcome by sexual desire for the beautiful thrall Yljali. As the war between the mighty kings Godred and Harald Greycloak escalates, Sigve finds himself fighting not only for his own survival, but for that of his people and of his love.