That was the Golden Dao Law Di Jiu had possessed at the start, which had escaped because Di Jiu had taken the Universe Membrane fragment.
Di Jiu had absolutely no idea that the Golden Dao Law had escaped at the moment. However, even if he had known, he would have continued using the Universe Membrane fragment to attain nirvana.
Although it was true that the Golden Dao Law helped Di Jiu's cultivation, the Dao Law remained in his consciousness without the intention of submitting itself to him. The two parties would cooperate if they matched well and split if they did not fit well. There was no need to cling onto each other.
The Universe Membrane fragment helped Di Jiu's physical body undergo nirvana. Di Jiu's body was destroyed by the Heavenly Fire Spirit before it recovered. Then, it was destroyed again and it recovered once more…