Descent to the underworld is when someone goes down to a place where we imagine dead people live. People used to believe that when someone died, they went to a place called the underworld. It was a scary place that you couldn't get to unless you went through a special door.
Sometimes in stories, people would want to go to the underworld to talk to dead people or get something important. In order to do that, they had to take a long journey. They had to walk a lot and follow some rules. They also had to meet some creatures along the way.
In some stories, people had to go through dark caves and dangerous forests to get to the underworld. They also had to cross a big river which might have been really scary. There might have been a scary guy there to take you to the underworld.
But if someone was brave enough and followed all the rules, they could get to the underworld and do what they needed to do. It was like a challenge that people had to complete.