Complex sentence: He was the illegitimate son of Grimoald II and Theodelinde of Frisia and thus a grandson of king Radbod, but his grandmother Plectrude tried anyway to have him recognised by his grandfather as the legitimate heir to all the Pippinid lands, instead of the illegitimate Charles Martel.
Split into simpler sentences: He was the illegitimate son of Grimoald II and Theodelinde of Frisia and thus a grandson of king Radbod. His grandmother Plectrude tried to have him recognised by his grandfather as the legitimate heir to all the Pippinid lands, instead of the illegitimate Charles Martel.