Why do they want Dunstan’s services? Why does he accept?

At lunch the next day, Dunstan learns that Liesl is more than just an ugly face. He recognizes in her both a keen intellect and a shrewd business savvy. She turns out to be a student of hagiography, and has read all of Dunstan’s books on saints. During the lunch, she makes him a proposal: they will pay him to write a false, mythical autobiography for Magnus Eisengrim, using as much artistic license as he likes. They want to increase Magnus's mythic appeal. Dunstan agrees because he cannot resist the adventure of it.

At fifty, should adventures come at all? Certainly that was what I was asking myself a month later. I was heartily sick of Magnus Eisengrim and his troupe, and I hated Uselotte Vitzliputzli, which was the absurd name of his monstrous business partner. But I could not break the grip that their vitality, their single-mindedness, and the beautiful mystery of their work had fastened on my loneliness.