Darkness
The plague of darkness had many strange aspects to it. Usually, darkness is described as “the absence of light.” The Torah describes the plague as a situation where וימש חושך, that this darkness was felt. This is why the Egyptians were unable to drive it away.
Another unusual aspect of this plague was that since the darkness enveloped the evil Egyptians, the light experienced by the Jewish people, was as brilliant as the light was on the first day of creation.
This led the Chidushei Harim to make an amazing observation. He commented that the worst possible darkness is when an individual is unable to feel the pain of his friend. His failure to reach out to help, paralyzes him in a way that was similar to the darkness felt by the Egyptians. The darkness was so thick that they were unable to move. It is also understandable why this was the appropriate time to eliminate the unworthy Jews from being able to leave Egypt.
It is a terrible thing when people to choose to live in darkness, and are unable to distinguish between good and evil, and right and wrong.
It is clear why darkness was the ninth of the Ten Plagues. There is so much to be learned from this unusual plague.