The cycle that Jiang Chen used to replace the Heaven array, was the Star Array. In the past, when the Demon Clan ruled the heavens, this Star Array was indeed the best interpretation of the heavens.
The one he used to replace the Earth array was the Dutian God-killing Array. The Demon Clan ruled the heavens while the Witch Clan managed the earth. This Dutian God-killing Array was indeed the best representation of the earth.
To combine the power of these two supreme arrays and reconstruct the Innate Heaven and Earth Great Formation, this formation's power would be unimaginably formidable, worthy of being called the number one supreme formation in the Prehistoric World.
Therefore, it was not surprising that Jiang Chen failed.
If these two supreme arrays were easy to merge, people would have done it long ago, back in the ancient times.
