Remnants of Bregonne, also known as The Scourge of Aquitaine, is a Rogue army faction introduced in Total War: Warhammer II. It is led by The Scourge of Aquitaine, a Vampire Lord and can be found roaming in both campaigns.
Their army is composed of heavy Bretonnian and Vampire Count cavalry.
The Remnants of Bregonne Rogue Army is inspired by Marcel of Parravon. He was a lord who uprooted his household and sailed to Lustria, founding the settlement of Bregonne. There, he raided artifacts from some Lizardmen, who attacked to regain their artifacts. This army is the remnants of that town, led by the Bretonnian on a quest for vengeance. As such this is likely a bug, as there is no lore reason for Bregonne to be associated with Vampires.