Boo

From Paper Wiki

Boo is a ghost like creature that has the power to become invisible,posession,and joining together to create Atomic Boo.In PMTYD they are found in the creepy steeple.In the creepy steeple,there is a boo who won't let mario out,unless you find a secret passege under the steeple where his friends are locked up in a chest.When you open up the chest,200 boos fly out.If you answer the question right(answer is 200 boos),they will give you an ultra shroom.