The Extraordinary Homunculus of Baron von Orfen is a book written by Helen Flannel in Fable III.
Baron von Orfen was a dabbler in the black arts, particularly alchemy. As a young nobleman, he applied a sample of his own blood, mixed with various alchemical substances, to a mandrake root and created a small, diminutive version of himself -- a homunculus. It was his dark aim that this creation would become a vessel for any ailment or malady which would otherwise plague the Baron himself. From that day forward, Baron von Orfen suffered no disease, no wound, no wart, nary any affliction at all. Thus it was that the Baron lived well past the age of eighty, until one day he accidentally knocked the homunculus into an alembic of powerful acid. There it promptly dissolved, and the horrified Baron, seeing this, steeled himself for the worst...but was fine. Turns out he just had a really amazing constitution.