Tafsir of Al-Muzzammil 73:16

But Pharaoh disobeyed the messenger mentioned, whom We sent to him. The definite article is for a previous mention (al-‘ahd al-dhikri). The kaf (in ka-ma arsalna) is in the accusative position as an adjective for an omitted verbal noun, based on the assumption of it being nominal—meaning: a sending like Our sending. Or, the preposition and its object are in the position of an adjective, based on the assumption of it being adverbial—meaning: a sending existing as We sent.

The meaning is: We sent to you a messenger as a witness over you, and you disobeyed him, just as We sent a messenger to Pharaoh, and he disobeyed him. In repeating the nouns "Pharaoh" and "the messenger" explicitly, there is an intensification of the gravity of his disobedience, highlighting that it was disobedience to "the Messenger" qua messenger, not merely disobedience to "Musa."

Furthermore, this implies that the disobedience of the [current] addressees is more heinous and more deserving of condemnation, for this Messenger has been elevated with an additional attribute—namely, "a witness over you"—and within this is an implication that had they believed, the testimony would have been in their favor.

His saying, Exalted is He, So We seized him with a heavy seizure (akhdhan wabila): that is, burdensome and of evil consequence, derived from the expression "a wabil pasture" or "food that is not digested due to its heaviness." Al-wabil also signifies a large staff; from this comes the word al-wabil for rain with large, heavy drops. This is not merely a simile; it is brought forth to warn the addressees that they shall be seized with the like of that, and even more severely.