the first clue is steps from Jasper... Each time he (or any other NPC part of the quest) says some, one of the words (or the abbreviation FOA for the Followers of Armegeddon) will be a code word for additional info. So, if Olivia is mentioned, say her name, and he'll tell you the town she's in, and that info might have further prompts for details.
You start by going in to Francesco's corpse, and use Forensic Evaluation (works even at 0 skill) on the corpse until you find the clue, get the clue, and bring it to Jasper. from there, it's a matter of following the prompts from the others.
There's detailed walkthroughs on several sites, already - and note that Brian's journal elsewhere in the castle is a red herring - as is the unreachable Aileen under the castle (that's just where they stuck the normal NPC to be out of the way for the event).
When you do get to Trinsic, via the clues, note that a yellow paladin in the NE part of town (not on an island) is the step after following all your leads on Aileen.
