Marcellus Pye was the last Alchemist and a renowned Physician who lived in the castle five hundred years in the past. He had discovered a Tincture that grants eternal life but not eternal youth, causing him still be alive while decaying and being horribly sick in Septimus Heap's time.
Septimus Heap traveled back in time through a glass made by Marcellus and became the Alchemist's apprentice. Once Septimus went back to his own Time, he made a new Tincture for the 500 years old Marcellus so he can be young in Septimus's time, too. When Septimus had returned to his time, he made a new Tincture for Marcellus, which reverted him to the age of 30. Marcellus then started living in Snake Slipway.
His name was accidently spelled "Pie" by Terry Tarsal, who made shoes for him.