he heard say concerning Tirhakah king of Ethiopia, He is come forth to make war with thee. And when he heard it, he sent messengers to Hezekiah, saying,
וַיִּשְׁמַ֗ע עַל־תִּרְהָ֤קָה מֶֽלֶךְ־כּוּשׁ֙ לֵאמֹ֔ר יָצָ֖א לְהִלָּחֵ֣ם אִתָּ֑ךְ וַיִּשְׁמַע֙ וַיִּשְׁלַ֣ח מַלְאָכִ֔ים אֶל־חִזְקִיָּ֖הוּ לֵאמֹֽר
ἐξῆλθεν Θαρακα βασιλεὺς Αἰθιόπων πολιορκῆσαι αὐτόν καὶ ἀκούσας ἀπέστρεψεν καὶ ἀπέστειλεν ἀγγέλους πρὸς Εζεκιαν λέγων
2 Kings 19:9
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
וַיִּשְׁמַ֗ע אֶל־תִּרְהָ֤קָה מֶֽלֶך־כּוּשׁ֙ לֵאמֹ֔ר הִנֵּ֥ה יָצָ֖א לְהִלָּחֵ֣ם אִתָּ֑ךְ וַיָּ֨שָׁב֙ וַיִּשְׁלַ֣ח מַלְאָכִ֔ים אֶל־חִזְקִיָּ֖הוּ לֵאמֹֽר
καὶ ἤκουσεν περὶ Θαρακα βασιλέως Αἰθιόπων λέγων ἰδοὺ ἐξῆλθεν πολεμεῗν μετὰ σοῦ καὶ ἐπέστρεψεν καὶ ἀπέστειλεν ἀγγέλους πρὸς Εζεκιαν λέγων
I going to assume Isaiah and Kings are unoriginal and both a drawing from a Q source. The Septuagint version of Isaiah is so distant compared to the Hebrew. l, it appears Septuagint is translating from an unknown version of Isaiah.
The Greek Isaiah as ἀπέστρεψεν, a common reading for וישב which is absent in Isaiah but present in Kings.
Greek Isaiah as πολιορκῆσαι αυτόν, an infinitive aorist verb meaning "to besiege, to be blocked" that grammatically matches להלחם and Kings as πολεμεῗν, a dative, the ה suggests a preposition, cf. ἐκπολεμέω.
The word כוש is interpreted as Ethiopia in both Greek texts.
Σκυθία = כוש : Scythia
Σκυθιας : a name for Delos
The Scythian rivers (נהרי כוש); the Euphrates and Tigris
Σκυθών = כושן
Σκυθιας = כשית