The penultimate plague was Darkness. There are many explanations offered as to why this is. One of them, the Medrash explains is that the darkness was 'mida keneged mida' (measure for measure) for the acts of the Egyptians against the Jews. It is said that Egyptians would eat their evening meals with a Jew standing by them with a candle of his head - and the threat of death if he were to move. Therefore we can now undetstand the possuk which says that "no man could see his brother nor could anyone rise from
his place..."(Exodus 10:23). Just as the Egyptians would not let their Jewish slaves move, HaShem was showing them who the real 'master' was by not allowing them to move.