In 787, he and his elder brother Romoald were sent as hostages to Charlemagne who had descended the Italian peninsula as far as Salerno to receive the submission of Benevento.

In return for peace, Arechis recognised Charlemagne's suzerainty and handed Grimoald over as a hostage.

He recognised Frankish overlordship, but was permitted practical independence in return for defending Italy from the Eastern Romans.

A Frankish army under Winigis and Hildebrand, Duke of Spoleto, joined Grimoald and defeated Adelchis on the coast soon after his landing.

Later, Grimoald tried to throw off Frankish suzerainty, but Charlemagne's sons, Pepin of Italy and Charles the Younger, forced him to submit in 792.