זוהר שמות — דף יח ע”ב

Idol worship was abundant in Egypt and the Nile was their main god. The Eternal executed judgments against all their gods and against the Nile first. The celestial minister of Egypt was struck first, then the other gods, as it is written: the blood was in all the land of Egypt and in the trees and the stones, for they were actual divinities for them. Rabbi Yohanan said that the waters of the Nile had their counterpart in the celestial heights, which is why their minister was struck first. The punishment of the people of Egypt was the most terrible of all, as their death was cruel and brutal.