How to get the "easy to miss" new encounter with Agnimitra & the Harpy Matron?

[Papum]

"as well as an easy-to-miss one-time meeting with the Harpy Matron, if you manage to go through the effort to recruit Agnimitra before going too deep into the Foothills."
Tried to do this and didn't went beyond the first blue zone as i assumed the part of "...going to deep..." would be entering the second and third parts of the zone but when i managed to recruite Agnimitra i cleared the zone completly and got no such event, only event that triggered was a encounter with Zhara and Agni.

 

[valk42]

enter foothills for the first time with agni in party
it's the encounter where you get ensorcelled by her

 

[Papum]

enter foothills for the first time with agni in party
it's the encounter where you get ensorcelled by her

They could have specified it better instead of what was written "...before going too deep into the Foothills." ... . That's the literal entrance.