One of the two is certainly Vormav's alterego Hashmalum, as the News reports that an unidentified monster at Riovanes Castle caused over 500 deaths. It's difficult to say, really, which of the other Lucavi demons caused the most carnage. I would suspect, though, that it was Queklain; the deaths that occurred at Lionel Castle were attributed to "crush[ing]," which is consistent with the deaths caused by Hashmalum, and suggests that a Lucavi demon was responsible.
So, Hashmalum and Queklain.