Hi morgain!!!
Seems to me that I recall killing the "beast" during the day. The trigger, as I recall, is finding the writ on the burnt body (between the Spire and the Mines in an inlet in the forest - map in Best Tips thread blah blah blah).
Look for an idol at the "end" of the Boogre Caves to set your party right again! (not the one with all the fire and nifty rewards to hand out if you give it Boogre Brew - you did that, right??).
Hope you are making progress!!
Wyv