Merlin's ability to touch the edge of the Original Law meant that he had a chance to access Solomon's Original Law. Moreover, the rules of the law were rushing into his head, making his cultivation increase rapidly!
The Original Laws were ancient laws that a living thing could not create.
In other words, the origin of this law would have been Solomon inheriting it.
The war ended because Solomon had summoned a Great God to come down to this world. This meant that he had quite a good relationship with that Great God. It was possible that this Original Law belonged to that Great God or the other Great Gods he knew.
Either obtaining inheritance or self-enlightenment was equally difficult. Not all of the Gods inherited the laws from the ancient beings could become Great Gods.
Within this universe, there were not many Great Gods. Each of them ruled a different galaxy. And it was rare that such a great being would intervene in a world.