Petronella & The Trogot

Petronella finds out that the hole is a deep pit and is terrified to go near its ridge. But one night...

How is the tree linked to:-

* The creepy night-time visits by the Hooded Horseman?

* The boy ghost, Percy, Petronella finds in her bed one night?

* The invasion of ghosts of the ancient Strincas population all over Fort Willow?