According to my theology professor, Hades and Hephaestus used to be the same deity in Greek Mythology and were only later split into two.
I thought about this and it makes a lot of sense - Hephaestus was rejected by his mother Hera for being ugly and lame and banned from Mount Olympus.
Hades is the god of the underworld.
Considering that these were initially the same deity, it would explain how Hades got into the underworld in the first place: he was banned there.
It's very similar to the story of Lucifer, the angel who was banned from heaven by God and became "the ruler of hell".
Every time somebody enters some dark place under the ground, it's a symbol for entering the unconscious mind.
This is a common mythological theme.
Joseph Campbell said that "deities who are not recognized become demonic". In a similar vein, Freud said that "unexpressed emotions will never die. They are buried alive and will come forth later in uglier ways."
Mythology is a metaphor for the psyche - whatever is repressed will turn against you, whether it's a god, an angel, or a psychological complex.
Things turn evil after being rejected. Just think of basically every villain and their backstory.
I wrote more about this in a blogpost on Pluto in Myths and Stories, if you're interested.
That is why outcasting and demonizing criminals is counter-productive - it turns them into even stronger antagonists of society.
Carl Jung came up with the shadow - all contents of our psyche that we suppressed into the unconscious, because we thought they were unacceptable (maybe ugly and lame like Hephaestus? Rebellious like Lucifer?) - those can be both bad and good/neutral traits!
We might suppress anger, because we grew up in a family or culture (e.g. Christianity) that taught us that "anger is bad!" and punished us, judged us, blamed us, for showing anger.
As little children, our survival depends on being loved and accepted - literally.
So in order to survive, we mustn't show anger! We suppress it.
What happens then is that we don't find a healthy outlet for it, become passive-aggressive, and instead of expressing our upset immediately, turn our anger into sick forms of revenge, power games, etc.
The emotion that was buried alive came forth in an uglier way.
One might deny it and say "I'm not angry!", and it will be true, because we really are dis-identified from our shadow content and not aware of the destructive ways in which they express themselves.
The way to become whole again is to make the unconscious conscious.
To find and then accept the parts of you that you once suppressed and rejected.
That is what Shadow work is about.
If you want to learn more about Shadow work, I recommend the YouTube videos by Teal Swan and Kelly-Ann Maddox, the blog Unmasking the Divine, the book Why Do I Do That by Joseph Burgo and just a general openness to facing contents of the unconscious mind and realizing how you have been lying to yourself to avoid pain.