Complex sentence: Guy is murdered on the Tiber River by agents of Alberic I, a Frankish nobleman with political interests, seizes Spoleto (possibly at the instigation of king Berengar I) and sets himself up as duke.
Split into simpler sentences: Guy is murdered on the Tiber by agents of Alberic I, a Frankish nobleman with political interests. He seizes Spoleto (possibly at the instigation of King Berengar I) and sets himself up as duke.