Complex sentence: Of the listed tribes, only the "Frisii" (normally read as "Frisians") and "Chauci" are well-known from other sources, as ancestors of the later Saxons inhabiting a large part of northwestern Germany, north of the Rhine.
Split into simpler sentences: Of the listed tribes, only the "Frisii" and "Chauci" are well-known from other sources. The Chauci were ancestors of the later Saxons, and inhabited a large part of northwestern Germany, north of the Rhine.