So

So is the king of Egypt to whom Hoshea, the last king of the northern kingdom of Israel, secretly turned for support against Assyria. The appeal precipitated Hoshea's downfall.

King of Egypt

Hoshea's withheld tribute and his clandestine embassy to Egypt are reported together as the conspiracy that triggered Assyrian reprisal: "And the king of Assyria found conspiracy in Hoshea; for he had sent messengers to So king of Egypt, and offered no tribute to the king of Assyria, as he had done year by year: therefore the king of Assyria shut him up, and bound him in prison" (2Ki 17:4). The verse is the only place the name appears, and it stands at the hinge of the narrative — the immediate cause given for Hoshea's imprisonment and the larger campaign that ends with Samaria's fall in the following verses.