Drum Billet is (or rather was) a wizard, and like all wizards he knew his end was coming. So he went in search of someone to pass his powers unto, and it led him to Gordo Smith, Bad Ass's smith, who was an eighth son, had seven sons, and whose wife was to deliver an eighth child. So he passed his staff to the newborn, thus (unknowingly) making Esk the first wizard woman (which caused problems in the slightly sexist wizarding community). After Billet had died, he spoke to Death, learning of the loads of time it takes to be able to reincarnate into a human.
He then got reincarnated in an apple tree in the Smith's backyard, and watched over Esk. The tree produces Green Billets, apples famed for being almost impossible to eat, but ideal for making scumble. After a while as a tree, he got reincarnated again as an ant, coincidentally, in Unseen University.