Canthan cartog: west Zu Heltzer

Hi there,
I'm close to the end of my Canthan's title, but one small part, located on the west of Zu Heltzer, give me some headaches.

I tried to discover this area from differents sides, from House of Zu Heltzer, from Altrumm Ruins, and from Arborstone, but nothings happened.

Here is the problem:



Could anyone help please? =)

go to altrumm ruins and fill your party with heroes/henchmen, click enter battle and run around

Its part of the Altrumm ruins challenge mission.