How Did Coruscant Become Hell After The Fall Of The Empire?

Coruscant got hit four times in a row. Imperial warlords tore the leadership apart after Palpatine's death, the Yuuzhan Vong conquered the planet and terraformed it into a hostile jungle, the reconstruction that followed left the lower levels sealed in the dark, and then the entity Abeloth arrived and set off seismic devastation across a large portion of the world. All of it is Legends. (X-Wing: Wedge's Gamble; The New Jedi Order: Traitor; Jedi Search; Fate of the Jedi: Apocalypse)

The warlord years#

The rot started immediately. In the six months after Palpatine's death, New Republic operatives destroyed several Coruscanti HoloNet installations that were critical for communication between the capital and Imperial forces across the galaxy. With no orders coming from Coruscant and no clear successor, Moffs and admirals in outlying regions became warlords, seizing sectors and fleets for themselves. Admiral Gaen Drommel simply left with a Super Star Destroyer to defend his home sector, Oplovis. (The Essential Atlas)

On the planet itself, Grand Vizier Sate Pestage took control and was immediately opposed by Palpatine's old advisors. Ysanne Isard played the neutral broker between the two while quietly pitting them against each other, and took the world for herself in 5 ABY. (X-Wing: Wedge's Gamble)

Her method of holding it was worse than the infighting. She had a team of scientists create the Krytos virus, which only affected non-Human species, and contaminated the city planet's water supply with it. When Rogue Squadron and Ackbar's fleet finally took Coruscant, millions of its non-Human population began dying within days. Isard escaped aboard the Lusankya, and on the way out the ship devastated over 100 square miles of surface and killed millions instantly. (X-Wing: Wedge's Gamble; X-Wing: The Krytos Trap)

That was the warm-up.

The Yuuzhan Vong turned the capital into a jungle#

The extragalactic Yuuzhan Vong seized Coruscant in 27 ABY. The battle cost them dearly, but they took it, and the New Republic fragmented in the process. (The New Jedi Order: Star by Star)

What they did next is the part people remember. They terraformed the planet in the image of their own lost homeworld and renamed it Yuuzhan'tar. The Manarai Mountains, the last part of the planet untouched by cityscape, were blasted into giant craters. The Western Sea, at that point the planet's only body of water, was turned into an enormous succession pool. The Senate Building became the site of the World Brain, a bred creature called a dhuryam whose entire purpose was managing the thousands of biological processes needed to run the terraforming. (The New Jedi Order: Traitor)

Thirteen dhuryams were grown for the job under Master Shaper Ch'Gang Hool, and one was selected. The seat of galactic government became a nursery for an alien biosphere. By then the Jedi Temple was a ruin, the Senate was wrecked and orbital bombardment had broken the city, and the capital was becoming an alien jungle. (The New Jedi Order: Star by Star; The New Jedi Order: Traitor)

The video's shorthand is that they filled it with ferocious animals and plants that preyed on the citizens. That is what a Vongformed world was for. (The New Jedi Order: Traitor)

The end was worse than the invasion. Believing he was going to lose the final battle, Supreme Overlord Shimrra ordered the World Brain to start a chain of fires and earthquakes that would ultimately render the planet uninhabitable. He convinced it to burn the sections already broken by groundquakes and magma streams. Galactic Alliance forces retook the planet in 29 ABY, but the damage was done. (The New Jedi Order: The Unifying Force)

The rebuild only went upwards#

Coruscant was rebuilt as the capital of the Galactic Alliance, returning to the ecumenopolis style wherever possible and leaving the Yuuzhan Vong terraforming in place where changing it was not yet feasible. The World Brain stayed in the Senate Building, because there was no way to remove it without causing further damage. (The New Jedi Order: The Unifying Force)

This was not the first time the planet had been rebuilt from rubble. After Palpatine's final death in 11 ABY at the Battle of Onderon, the New Republic had used EVS Construction Droids to clear debris and throw up new gleaming skyscrapers. (Empire's End; Jedi Search)

Both times, the reconstruction had the same blind spot. The shadowy lower levels remained unaffected by the programme and stayed the haunt of crime syndicates. Many still struggled to survive in the lower forty or fifty levels. Building upward on a world that is already solid city means everything underneath gets buried a little deeper. (Jedi Search; The Jedi Academy Sourcebook)

That is what the video is pointing at when it says the droids cut whole levels off from the surface. Every rebuild added another layer of roof. (Jedi Search; The Jedi Academy Sourcebook)

Then Abeloth showed up#

The last disaster was the strangest. Abeloth, a dark side entity from the Fate of the Jedi series, took power on Coruscant by presenting herself as a candidate for its leadership, and in doing so led the Sith onto the planet. (Fate of the Jedi: Ascension)

Getting rid of her cost the world again. A large portion of Coruscant was devastated by seismic activity Abeloth caused as a means of escaping the Jedi. After her defeat, Wynn Dorvan became the next Chief of State. (Fate of the Jedi: Apocalypse; X-Wing: Mercy Kill)

Four catastrophes, roughly seventy years, on a planet where the entire surface is a single city and there is nowhere for anyone to go.

Where it ends up#

There is a coda that softens it slightly. A painting titled Green Coruscant, created sometime after 104 ABY, shows a populated area of the planet full of floating structures and greenery. Whatever was left of the Vongforming, somebody eventually made peace with it. (Celestia Galactica Photografica)

None of this is Disney canon. The warlord era, the Yuuzhan Vong War and Abeloth all belong to Legends, spread across the New Jedi Order, Legacy of the Force and Fate of the Jedi novels. In the current continuity, Coruscant's story after the Galactic Empire falls goes somewhere else entirely. (The New Jedi Order: Star by Star; Fate of the Jedi: Apocalypse)