The Gardener Ghost (real name Ed Lestwit) is a headless undead creature (presumed formerly human) who is found at the back of Fenkenstrain Castle. He permanently carries a spade.
Ed was the gardener for Fenkenstrain Castle, and lived there along with the castle's doctor, Dr. Fenkenstrain (whom Ed fondly refers to as "old Fenky"), Lord Rologarth and the other "castle folk". The others gradually "disappeared", and Ed was the one who dug their graves until only he, Dr. Fenkenstrain and Lord Rologarth were left.
Ed's account of his own death was murder by decapitation whilst digging a pit in the Haunted Woods. Dr. Fenkenstrain had ordered him to dig the pit, though he "never said what for".
The implication is that Dr. Fenkenstrain was the one who killed him, having killed everyone else already. The pit that Ed was digging was his own grave, as Dr. Fenkenstrain murdered him in it.