Okay, I found a person in GOG's forums with a similar problem as mine, here it is:
"Here is the situation:
Had Sebille with me but kept her away while Red Prince lied to the Shadow prince to get Sadha s location.
No fight, the shadow master leaves.
forward to Arx, I Red Prince becomes the father of dragons, Sadha is alive.
After my party comes back to reality the Shadow Prince shows up and attachs.
The sketchy part is at the end of that fight :
No update from Sebille about the fact that the Shadow prince only showed up now and is now dead (she is free)
The log seems to not have recorded that either.
Another thing telling me it may be a bug:
The ghost of the Shadow prince is there but says nothing.
Bug?"
Is there a way to progress the quest from here?
