What does verse 6 mark the start of?

Ezra 4:6 (ESV)

6 And in the reign of Ahasuerus, in the beginning of his reign, they wrote an accusation against the inhabitants of Judah and Jerusalem.

Here the author begins a parenthetic section in which he recounts events that took place a long time after the events described in Ezra 4:1–3. He shows how the opposition to the Jews was seen not just in the reign of Cyrus and Darius, but also in the reign of Xerxes. The author does this in order to show how the rejection of these people’s help (Ezra 4:3) was justified by the actions of these people in history (continuous opposition).