After the completion of Hunting the Archon there is an email from Moshae Sjefa titled "Wisdom" inviting Ryder to meet some angarans who still remember how it was to fight the kett when they arrived. Acknowledge it to start the mission.
Go to to Aya and speak to Daanesk, Anaret, and Vuhlren. They were all children when the kett arrived.
Anararet tells you about how the kett first pretended to be friends, then show their true nature and create fear in the angaran making them hesitate. That's when the kett struck.
Daanesk teels you about how the kett just after they have arrived used a false signal to lure his shuttle into a trap. The shuttle's captain sensed something was wrong, and turned back instead.
Vuhlren tells how the kett lured then by pretending to give the angarans what they wished. Then the council dissapeared, and the town fell into chaos.