In the fictional Babylon 5 universe, Z'ha'dum was the homeworld of the ancient, mysterious race known as the Shadows. Inhabitants of this world are known as Z'ha'domites. The planet is near the galactic rim. The Shadows would return to the world every time they were defeated.
The surface is mostly desolate, with raging dust storms lashing the rocky surface. Stone obelisks covered in Shadow Runes pierce the land. Roughly translated, they say "Every light carries a shadow." The obelisks, made of native sedimentary rock, range from 100 to 150 yards high. They are spaced at a constant 2.43 miles apart and are spread over large parts of the planet, and appear to be concentrated in areas near cave entrances where Shadows dwell. Any other structures that were on the surface, however, did not survive.
The Icarus, a science vessel from Earth, found the Shadows and inadvertently woke them from their long hibernation. After this the Shadows gave the humans a chance to agree to work with them. Morden agreed to do so, and became the liaison between the Shadows, the Centauri Republic, and President Morgan Clark's government on Earth. Others who refused, like Anna Sheridan, were either killed outright, or were taken and plugged into Shadow ships to act as its central processor. The Shadows built a complex on the surface to accommodate the humans who agreed to assist the Shadows.
When the Shadows learned that Anna Sheridan was plugged into a Shadow vessel, they removed her and sent her to Babylon 5 to bring John Sheridan back to Z'ha'dum in the episode of the same name. Sheridan discovered Anna had been plugged into a Shadow vessel, and that her personality had been destroyed in the process. When the Shadows tried to attack him, he shot at the approaching Shadow, and went to a balcony overlooking the Shadow capital city. He set the White Star to ram the city, and set the two nuclear devices on board to detonate Upon hearing the voice of Kosh, Sheridan jumped into the great pit below the balcony. The White Star exploded, killing Anna Sheridan and destroying the Shadows' capital city.
During his conversations with Lorien, Sheridan learns that he is stuck in between life and death. Eventually, Lorien convinces Sheridan to let go of his life and then manages to restore Sheridan's broken body, effectively bringing him from the dead (Lorien´s power is vast, although not unlimited; he says he cannot create life anew, but he can revive the recently deceased, apparently with the flick of a thought). Lorien then surprisingly returns with Sheridan to Babylon 5, after not leaving Z´ha´dum for what was presumably a million years.
Once they arrive, Sheridan learns that Delenn has been staging a massive attack against Z'ha'dum. He arrives on Babylon 5 to find support wavering, and a large demonstration against Delenn's plans underway. This dispute is resolved when Sheridan suddenly walks in on the gathering, in fact, most of the ambassadors put on dumbfounded expressions they certainly wouldn´t like having on a picture. As no one has, ever, returned from Z´ha´dum alive while it was under Shadow control (even Kosh the Vorlon considered the trip plain suicide), Sheridan gains an almost godlike status in doing so (no matter the hard hit he´s dealt the Shadows with the nukes) and most in the Army of Light now seem to believe him to be a guarantee of victory, much like their very own Ark of the Covenant.
Drazi Ambassador: Captain Sheridan! We thought you were dead! Captain John Sheridan: I was. I'm better now.
Sheridan is able to trick the Shadows and Vorlons into revealing their true intentions to the younger races - that being that the younger races under the two titans´ respective influence fight each other in a war of ideology, the advancement through order of the Vorlons versus the strength through chaos of the Shadows, until one of them proves the other right, presumably by defeating all of the other´s accolytes. The Shadows and the Vorlons were long past having any real hostile intent against each other and have systematically, over tens of thousands of years, reduced themselves to bickering parents, as Sheridan put it, with countless infant races paying the price for their pettiness. Sheridan and Lorien are able to convince the two races to go beyond the galactic rim to join the other First Ones who had left eons before. Ashamed, the Shadows and the Vorlons accept, and Lorien and the other First Ones also leave the galaxy, leaving it to the purview of the younger races. Apart from Lorien alone returning to bring Sheridan back to the other First Ones in 2281, no First One has ever been seen in our galaxy again.
Upon seeing the evacuation, Sheridan realized something was very wrong. He had the crew turn the ship around. Within a couple minutes, the planet exploded. After their return to Babylon 5, Sheridan confronted Lyta Alexander in her quarters concerning the destruction of the planet. He all but accused her of triggering the planet's destruct system by telepathy. While she did not admit to trigging the self-destruct, either intentionally or unintentially, she stated that, hypothetically speaking, she might have for any one of several reasons with revenge being the motivator. She cited as reasons the Shadows killing Ambassador Kosh and the treatment of telepaths by Psi Corps in general and Alfred Bester in specific. Sheridan told her that if she ever decided to make such strategic decisions on behalf of Babylon 5 ever again, without consulting the command staff, there would be hell to pay.