Navigating the world so deep underground was a challenging task. Just being off by one degree would make them miss their destination by hundreds or thousands of years of travel.
After around 30,000 years, they began to triangulate their relative positions according to the flow and density of the Energy in the world.
By now, enough time had passed that Gravis and Mortis could create another clone due to their passive absorption of Energy.
Now, two Gravises and two Mortises flew around in the underground.
They all flew in slightly different directions and crosschecked the pull of Energy from their different positions, which gave them a general direction towards their goal.
In total, the journey took 50,000 years, just as Gravis had anticipated.
By now, the pull of Energy was astonishingly powerful. It was like the Energy of the entire world was just in front of them.
Shortly after, the four of them fused again.