Take the manuscript - you killed Cybu by yourself? Then go to Ralph, the commander south of the castle. Did he give you the quest to investigate the ducal murder?
Kiya
The invitation is a small blue piece of paper in your misc inventory, given to you by Ralph - please, check again.
![[Linked Image]](http://kiya1.via.t-online.de/images/Div_Invitation.jpg)
As Kashya said, it may be hidden...