Complex sentence: He is sometimes referred to as Valdemar III (his tombstone calls him Valdemar III, King of Denmark, son of Valdemar II), but that name is more commonly used to denote a later king, Valdemar of Schleswig.
Split into simpler sentences: He is sometimes referred to as Valdemar III for example his tombstone reads in "Valdemar the Third, King of Denmark, son of Valdemar the Second". Although Valdemar II name is more commonly used to denote a later king, Valdemar of Schleswig.