Complex sentence: Dudo's "Historia Normannorum" (or "Libri III de moribus et actis primorum Normanniae ducum") was commissioned by Rollo's grandson, Richard I of Normandy Dudo likely had access to family members and/or other people with a living memory of Rollo.
Split into simpler sentences: Dudo's "Historia Normannorum" (or "Libri III de moribus et actis primorum Normanniae ducum") was commissioned by Rollo's grandson, Richard I of Normandy. Dudo had access to family members and/or other people with a living memory of Rollo.