Judging from the battle traces left on the scene, it seems that this battle just ended not long ago. Combined with what the Netherworld God said, it appears her battle with the three realm kings lasted for at least more than two months.
Since the battle has only just ended, the residual forces left behind by both sides have yet to dissipate, still continuing the battle between their former masters. Different attribute energies are eroding each other, attacking each other.
Through these remaining forces, Lu Hong understood the Netherworld God's intention, which was to familiarize himself with the forces of the three realm kings through these traces.
Lu Hong didn't know how badly those three realm kings were injured; surely, they weren't much better off than the Netherworld God.
