Petras can first be encountered on the first floor of Fraygo's Flophouse in Wyrm's Crossing, where he is hunting with Dalyria. If Astarion is present, the three will have a confrontation about Cazador and his intentions for the ritual.
He will later be with the other spawn in Cazador's Dungeon, trapped and suspended while Cazador prepares for his ritual. Like the other spawn, Petras can be spared or killed after the ritual, or allowed to be consumed as one of its sacrifices.