What law are you referring to?
Can 6:6-9
The vice-regents and governors conspired together and then went to the king and said, “King Darius, live forever! We’ve convened your vice-regents, governors, and all your leading officials, and have agreed that the king should issue the following decree:
For the next thirty days no one is to pray to any god or mortal except you, O king. Anyone who disobeys will be thrown into the lions’ den.
“Issue this decree, O king, and make it unconditional, as if written in stone **like all the laws of the Medes and the Persians**.”
King Darius signed the decree.
The whole story here:
http://www.biblegateway.com/passage/?search=Dan%206&version=MSG