Rashi on Deuteronomy 28:59/רש"י על דברים כ״ח:נ״ט

1והפלא ה' את מכתך THEN THE LORD WILL DISTINGUISH THY PLAGUES — He will afflict you with plagues exceptional and quite different from other plagues.

2ונאמנות (lit., trustworthy, reliable), i.e. certain to chastise you — certain to perform the task entrusted to them.