So in the Bible Encyclopedia - ISBE

so (co', although the Hebrew might be pointed cewe'; Assyrian Sib'u; Septuagint Segor, Soa; Manetho, Seuechos; Latin Sevechus; Herodotus (ii. 137 ff), Sabakon): In all probability the "Sabaeo" of Herodotus, the Shabaka, who founded the Ethiopian dynasty, the XXVth of Egyptian kings. His date is given as 715-707 BC (Flinders Petrie, History of Egypt, III, 281 ff), but we may suppose that before his accession to the throne he was entitled to be designated king, as being actually regent. To this So, Hoshea, king of Israel, made an appeal for assistance to enable him to throw off the yoke of the Assyrian Shalmaneser IV (2 Ki 17:3 ff). But Hoshea's submission to So brought him no advantage, for Shalmaneser came up throughout all the land and laid siege to Samaria. Not long after the fall of Samaria, So ventured upon an eastern campaign, and was defeated by Sargon, the successor of Shalmaneser, in the battle of Raphia in 720 BC.