The Green Knowe Chronicles by Lucy M. Boston is about a boy who ends up being sent to live at an old gothic manor. Strange things are afoot, however...
The titular children are ghosts that are trapped there. Although the children themselves are benevolent, there's a tree in the garden which the children called "Green Noah", inhabited by a demonic tree-spirit. The little poem about Green Noah is especially creepy:
Green Noah, demon tree,
Evil fingers can't catch me!
In the fifth book, when Melanie sends a curse to Tolly's grandmother, carved on the skin of a child's hand.
The maggots and snakes also count as this.
The scene in the sixth book, when the Saxons are slaughtering the Britons.