God’s Law is eternal. Sin is the breaking of God’s Law. Jesus died for our sins so we don’t have to. This allows God to be both Just and Forgiving at the same time.
When you see “The Law & The Prophets” in the Bible, “The Law” refers to the first 5 books of the Bible, and “The Prophets” refer to rest of the books of the Old Testament.
The Book of the Law (Genesis thru Deuteronomy) was written by Moses and placed beside the Ark of the Covenant:
Take this book of the law, and put it in the side of the ark of the covenant of the LORD your God, that it may be there for a witness against thee.
Deuteronomy 31:26
The Ten Commandments were written in stone by the finger of God and placed inside the Ark of the Covenant:
And he wrote on the tables, according to the first writing, the Ten Commandments, which the LORD spake unto you in the mount out of the midst of the fire in the day of the assembly: and the LORD gave them unto me.
Deuteronomy 10:4,5
And I turned myself and came down from the mount, and put the tables in the ark which I had made; and there they be, as the LORD commanded me.