Watch on 123Movies. Based on The King in Yellow by Robert W. Chambers, Cassilda's Song follows Cassie, a gifted but insecure art student who becomes obsessed with Sylvia, an enigmatic model whose presence seems to awaken something extraordinary in her work. What begins as creative admiration becomes intimacy, ritual, and obsession. As Cassie's art grows increasingly strange and symbolic, her desire to create something perfect begins to consume her.