Which hostage son of Arechis later became duke of Benevento?

Grimoald was the hostage son of Arechis who later became ruler of Benevento. In 787, Arechis submitted to Charlemagne and sent his younger sons, Grimoald and Adalgis, as hostages to guarantee the agreement. Grimoald was held at the Frankish court while Benevento remained under Charlemagne's supervision.

Arechis and his elder son Romuald died soon afterward. Charlemagne then permitted Grimoald to return to Italy and succeed his father as prince of Benevento in 788. He recognized Frankish overlordship at first, but retained substantial practical independence and defended the principality against Byzantine attacks.

Grimoald's reign was marked by repeated tension with the Carolingians. He resisted Frankish control and later attempted to throw off its suzerainty, but Charlemagne's sons, Pepin of Italy and Charles the Younger, forced him to submit again in 792.

The question's use of “duke” reflects Benevento's earlier political title, but Grimoald is more precisely described as a Lombard prince. Adalgis was his brother, while Romuald was the elder son of Arechis who died before Grimoald's succession.