Albert Barnes Commentary Daniel 6:15

"Then these men assembled together unto the king, and said unto the king, Know, O king, that it is a law of the Medes and Persians, that no interdict nor statute which the king establisheth may be changed." — Daniel 6:15 (ASV)

Then these men assembled before the king - The Chaldee here is the same as in (Daniel 6:6), “they came tumultuously.” They were insistent that the law should be executed, and they probably feared that if the king were allowed to reflect on it, his resolve would weaken, and he would release Daniel.

Perhaps they dreaded the effect of the pangs of conscience he might experience during the silence of the night, and they, therefore, came tumultuously to hasten his decision.

Know, O king, that the law ... - That is a settled matter about which there can be no debate or difference of opinion. It would seem that this was a point so well settled that no question could be raised regarding it, and, to their minds, it was equally clear that if this were so, the sentence must be executed without delay.