Katabasis, also known as the Journey to the Underworld, is a key part of the Hero's journey that is often reflected in some way you know. It can be as obvious as Orpheus going into the literal underworld to save the person he loves, or more subtle such as Luke's turning himself over to the Empire in the hopes of restoring his father to the Light Side.
Odds are you've had some form of katabasis in your games somewhere along the way. I know I have. However, I've never done it intentionally, and while the Katabasis is meaningful I've never really played into what that could mean. It's something I'm toying with changing in the near future, but it's also worth talking about.
To make it simple, and per the Wikipedia article, Katabasis is done to reflect several things. First off, the nature of being able to go into the realm of the dead without being dead yourself is a sign of the hero's specialness. Then there is the act of returning. The quest fulfilled properly, the hero returns with something that makes them stronger be it an ally, a salve for their wounds, or just a bit of wisdom or knowledge to keep them going.
It's this final aspect most katabasis-esque events in my games have been missing. Yes, people have gone to the underworld, and usually with reason, but it wasn't special or significant in any particular way aside from just something they did. And then...and then there's Neil Gaiman's influence.
In the Sandman comic there is a volume where Dream and Delirium go on a trip to find Destruction. All three characters are more than gods if you haven't read the comics. They're the anthropomorphic personifications of concepts. At the end of the volume, one of Dream's enemies says that because of his quest he had sealed his own doom. Afterall, one does not seek destruction so willfully and return unscathed.
Combined, and I think katabasis could in a way make a hero 'more special' if special is defined simply as 'other than mortal.' The realms of the dead are not meant for the living, and the various realms are frequently jealous hoarders of what is theirs. The dead are also often depicted as hungry for life. So what happens when a group of adventurers travels to the underworld? What does the realm take as it's due tax? What do they have to leave behind? And what does that mean for the rest of the game going forward?
It's not much in the way of advice I suppose, but playing with concepts and themes is a good way to give more meaning to upcoming sessions or a game as a whole and make it deeper. If you've played with this theme, how did it go? How did it play out? Were you happy with the results?