Pope Formosus
When Pope Formosus again asked Arnulf to invade, the duke personally led an army across the Alps early in 894. In January 894 Bergamo fell, and Count Ambrose, Guy's representative in the city, was hung from a tree by the city's gates. Conquering all of the territory north of the Po River, Arnulf forced the surrender of Milan and then drove Guy out of Pavia, where he was crowned King of Italy. Arnulf went no further before Guy died suddenly in late autumn, and a fever incapacitated his troops. His march northward through the Alps was interrupted by Rudolph I of Burgundy, and it was only with great difficulty that Arnulf crossed the mountain range. In retaliation, Arnulf ordered his illegitimate son Zwentibold to ravage Rudolph's kingdom. In the meantime, Lambert and his mother Ageltrude travelled to Rome to receive papal confirmation of his imperial succession, but when Pope Formosus, still desiring to crown Arnulf, refused, he was imprisoned in Castel Sant'Angelo.