Now Sennacherib had been warned about Tirhakah king of Cush: “Look, he has set out to fight against you.” So Sennacherib again sent messengers to Hezekiah, saying,
Rabshakeh, having delivered his message and received no answer (whether he took this silence for a consent or a slight does not appear), left his army before Jerusalem, under the command of the other generals, and went himself to attend the king his master for further orders. He found him besieging Libnah, a city that had revolted from Judah, Kg2 8:22.
Commenting on 2 Kings 19:8-19
when he heard say of Tirhakah . . ., Behold, he is come out to fight against thee, &c.--This was the "rumor" to which Isaiah referred [Kg2 19:7]. Tirhakah reigned in Upper Egypt, while So (or Sabaco) ruled in Lower Egypt. He was a powerful monarch, another Sesostris, and both he and Sabaco have left many monuments of their greatness.
And when he heard say of Tirhakah king of Ethiopia, Behold, he is come out to fight against thee: he sent messengers again unto Hezekiah, saying, (e) That is, Sennacherib. (f) For the kings of Ethiopia and Egypt joined together against the king of Assyria because of his oppression of other countries.