J.K. Rowling came up with the Dementor characters during an episode of clinical depression; substitute “Depression” for “Dementors” and you have an apt description:
Depression’s are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them… Get too near Depression and every good feeling, every happy memory will be sucked out of you. If it can, the Depression will feed on you long enough to reduce you to something like itself…soulless and evil. You will be left with nothing but the worst experiences of your life.
They don’t need walls and water to keep the prisoners in, not when they’re trapped inside their own heads, incapable of a single cheerful thought. Most go mad within weeks.
And, like the infamous Dementor’s Kiss, the depressed person feels: