Question: Who is Argishti I Of Urartu's grandchild?
Answer: Rusa I

The Urartian Kingdom was at its peak during his reign, campaigning successfully against several neighbouring powers, including Assyria. Sardur II was so confident in his power that he erected a massive wall at Tushpa (modern-day Van) with the following inscription: He was succeeded by his son, Rusa I. Argishti I , was the sixth known king of Urartu, reigning from 786 BC to 764 BC