Complex sentence: Rudolph's ally, a Carolingian himself, Count Herbert II of Vermandois, took Charles captive by treachery and the young Louis's mother took the boy "over the sea" to the safety of England, hence his nickname.
Split into simpler sentences: Rudolph's ally, a Carolingian himself, Count Herbert II of Vermandois, took Charles captive by treachery. The young Louis' mother took the boy "over the sea" to the safety of England, hence his nickname.