Reaper of Souls - Achievement Guide - Girl Talk

1. Start at Act 5.2 (Souls of the Dead) - speak with Eirena, and have the conversation "Westmarch" to begin things (this is not one of the conversations required for the achievement). After that, you can have the conversation "The Survivor" with her.
2. Start at the "Talk to the Mystic" quest checkpoint under Act 5.2 (Souls of the Dead) - speak with Eirena, and have the conversation "Abandoned."
3. Start at Act 5.3 (The Harbinger) - speak with Eirena, and have the conversation "Demon Magic."
4. Start at Act 5.4 (The Witch) - speak with Eirena, and have the conversation "A Voice from Afar."
5. Start at Act 5.6 (The Battlefields of Eternity) - speak with Eirena, and have the conversation "Pandemonium." This will open up her side-quest to find what happened to her sisters. Find your way to the spirits of Eirena's sisters*, defeat their demons, then make your way down to the Lair of the Prophet Level 2. Defeat Sister Lysa, and the conversation "Lysa" will be available.
6. Start at Act 5.8 (Angel of Death) - speak with Eirena, and have the conversation "A New Prophet" with her.

* As you find each of the first three of Eirena's sisters, be sure to click their spirit after they have departed. Doing this will drop the three lore-books required for the People Finder achievement (Sibyl's Memory, Melaina's Memory, and Raissa's Memory).
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0:00 - Westmarch
0:15 - The Survivor
0:44 - Abandoned
1:13 - Demon Magic
1:54 - A Voice from Afar
2:21 - Pandemonium
3:02 - Lysa
3:32 - A New Prophet