[Sever]

Casius Percius is the Imperial in charge of construction of Raven Rock. If you've killed both him and Falco (Percius' second-in-charge), then my guess is you've dead-ended the quests. I've only sided with Falco and i've never completed the RR quests so don't take my word for it. Look for clues in the journal entries as to what to do next. Failing that, look for notes or any other clues on their bodies. Failing that, talk to each and every person in Raven Rock (assuming you've got that far in the quests) and see if their dialogue offers up any clues. I've never heard of being able to dead-end the RR quests but it's been a while since i played it.