Why does Harry first encounter the Dementors in "Harry Potter and the Prisoner of Azkaban"?
They are guarding Hogwarts looking for Sirius Black.
<s>[INST] <<SYS>> Answer as briefly as you can to the following question. You are required to give a one-shot answer: <</SYS>> Why does Harry first encounter the Dementors in "Harry Potter and the Prisoner of Azkaban"? [/INST] They are guarding Hogwarts looking for Sirius Black. </s>