Visiting Sophie and Alferd
Saito was a little worried that the mercenary hired by the Crown Prince would be able to notice the irregularities within the dust cloud which was why he used the Illusion to make it seem as though there were no irregularities.
It was a rather week illusion which would have shattered the moment there was a magic change but since the mercenary didn't know about it, he wasn't able to see through the illusion. The reason Saito was confident in his illusion being able to successfully fool the mercenary was because it wasn't cast on the Mercenary directly but on the surrounding, which fooled his eyes. It was generally very difficult to detect an illusion being cast on the surrounding.
Saito flew at a very high altitude and gradually increased his speed. After they passed the city, he descended at a very high speed that he would seem like a shooting star dropping on the ground.