They first appear physically in Chapter 14 on Turn 9, where they are all Level 18 and equipped with Yotsmungand. Their powerful spells combined with their high magic makes them worth retreating from.
If the player goes to Chapter 21x and reaches Turn 30, a squad of the Schwarze Rosen will appear. They are even more powerful than before, as they are all Level 20 and equipped with Fenrir, Yotsmungand, and Rewarp. They are nearly impossible to kill due to their spawning point, so it is best to leave the map before they arrive.
Later in the game, after Leif and his army manage to infiltrate Manster Castle, the "Palace of Evil", in Chapter 24, Raydrik requests Veld's assistance, who is clearly disappointed in Raydrik's failure to eliminate Leif and thwart his progress thus far. Before Veld leaves, he decides to assist Raydrik one final time by granting him with a squad of Schwarze Rosen members to command, who are then fought and defeated in the chapter.