The Witch's Island is a area only accessable from the Traincar, itself only accessable from the Dense woods You can get the Witcheffect from here.
To get here you must trigger an event from the traincar. Upon entering the area you'll see many Nopperabou (you cannot interact with them). Down the bottom you'll find the start of the boardwalk. This stretches for several screens with many interesting formations in the water. On the other side is the Witch's island and a red-eye'd Kimajo
Toggle the Kimajo to get the Witch effect.