When Legion combined the memories of the underworld with the ability to appear anywhere through the Mindscape, they were able to locate the underworld, bypass its defenses, and appear directly within it.
At this time, the god of life has just begun to fight the god of death. The god of death was already at a disadvantage and was dying, so he surely couldn't put up a fight against them.
In fact, both the god of death and the god of life didn't know that they had appeared in the underworld. This was because the virtual body was a mental construct that couldn't be seen with normal eyes.
The god of death had been able to see them earlier because they had chosen to be visible to the world. But now that they had come to kill him for good, there was no need to play around anymore.
