When playing the demo, I always talked to Edmund the digger ghost. I was never able to finish this quest. Finally, after playing the demo several times, I happened to do the cells backwards and found the leg bone. Those feral rats should show up whether you've talked to Edmund or not. If the leg bone is in the inventory when first meeting Edmund, the Dark Knight should warn you, better not show him his leg if you want him to teach you. This way you won't miss out on Edmund's teaching.
