Petra is a (currently) unknown phenomenon. Even the correct spelling has yet to be ascertained. The first warning the party got of the existance of this phenomenon was Roland appearing near Njall whilst he was in bed and bellowing the words "Beware Petra!" before diappearing through unknown means, having stolen various components of the barbarian's gear, including a climbing kit and fifty foot of rope. This Roland was noticeably older than the present version, and wore an eyepatch over his left eye.
Despite nigh-endless hypothesis there has been no clear answer as to what Petra is, bar a potential threat. Njall has made no attempt to investigate the issue.