It was one evening at the Crystal Empire in the land of Equestria. Inside the Crystal Palace, the crystal guards were on their nightly patrol to make sure no intruder entered the castle. As they continued their patrol, creeping around them carefully without being detected was a black wisp of smoke. It came around a corner as if it was on the trail of something.

Suddenly it approached a door and slunk underneath it. Inside the room was the bedroom of Shining Armor and Princess Cadence, the rulers of the Crystal Empire. The wisp of smoke started getting bigger and darker, as a shadow cast over the couple. The couple groaned, until their horns started glowing their their respective magical auras. The two suddenly woke up, “Cadence?” Shining Armor asked in concern.

“Shining Armor, what is...” they suddenly looked up and gasped seeing the large shadow above them started taking the form of a familiar dark unicorns head that had a red curved sharp horn and a pair of glowing red eyes. The two ponies jumped out of bed and onto the floor.

“No, it can't be!” Shining gasped in horror.

“King Sombra?” Cadence gasped in fright.

“Correct, Crystal Princess!” Sombra's spirit boomed.

“No, this isn't possible. We saw you get destroyed by the Crystal Heart!” Shining reminded the former dark king.

“My body, maybe,” Sombra began, “But it could not extinguish my spirit!”

“Why're you here?” Cadence demanded.

“To take back was it rightfully mine. This kingdom.” he answered.

“The Crystal Empire is no longer yours.” Shining retorted.

“Not so long as we are here to protect it!” Cadence agreed, as the two stood side by side, ready to use their magic.

“We shall see about that!” Sombra's eyes started glowing, as he blasted them with red energy from his horn.

The two jumped out of the way, before Sombra's blast hit the floor putting a burn mark in it, “How'd he do that if he's just a spirit?” Shining asked in confusion.

“My new ghostly form has given me newer abilities,” the king explained, “Abilities I will use on you!”

Suddenly the doors burst open, and the crystal guards galloped in, “Your highness’s!” one of the guards called, before they skidded to a halt upon seeing Sombra's ghost.

“No! That can't be...” one of the guards gasped as they started reliving the memories of his reign of terror.

“Yes, me,” Sombra answered, “And I will once again resume my rightful place on the throne of this empire!” he declared. Suddenly he was blasted from behind by magic that went right through him, “GAH!” he growledfrom the feeling of the magic passing through him, and saw it came from the combined magic of Shining and Cadence.

“Our magic passed through him.” Cadence gasped.

“But he still felt it.” Shining noticed.

Sombra frowned, “Though I may be a spirit I can still partially feel the magic surge go through me. No matter you will never be able to defeat me now that you cannot truly harm me!” he burst a window in the room to pieces, before flying off.

The guards were in shock, as were the royal couple, “Shining Armor, what do we do now?” the princess asked her husband.

“I'm not sure, Cadence, but what I do know is Princess Celestia needs to know about this.” Shining stated, as he used his magic to levitate some parchment, a quill, and ink over to them.

The two sat down and began writing a message to Princess Celestia. When it was finished Cadence used her magic to transport the letter from the Crystal Empire, all the way to the kingdom of Canterlot. In said kingdom, Princess Celestia was in her chamber looking over some reports of her student in Ponyville, until a new message appeared. Celestia noticed the Crystal Empire crest, “A letter from Princess Cadence and Shining Armor. I wonder what's so important they needed to send me a letter at this late hour?” she wondered, as she used her magic to unravel the scroll to read it.

After reading the contents, the princess was in shock, “Oh, my goodness. This is serious,” she stood up and used her magic to write a letter back to the two, saying she would look into it and report back with news as soon as she could.

After she transported the letter, she began pacing around her room trying to determine what to do to help them, “Based on what they wrote, King Sombra cannot be subdued by magic alone since it passes right through him. There has to be some way of taking on a ghost,” she sighed, “Sadly I don't know of anypony in Equestria with that kind of power...” she trailed off before an idea popped into her head, “But perhaps there may be somepony who can who's not of Equestria. If Phoenix Wright from the other world was capable of proving Rainbow Dash's innocence that one time, perhaps there may exist somepony else capable of taking on a ghost. And I have just the thing to find such a being.”

She trotted up to a wall close to her dresser, and moved a little pony figure on it like a switch. Suddenly a picture on the wall opened up to reveal a compartment containing a crystal ball. She used her magic to levitate the crystal ball and placed it in the center of the room on top of a table. She used her magic to activate it and spoke, “Crystal of Starswirl I call upon your power. Show me one not of this world with the power to take on the paranormal.”

She looked into the crystal and saw multiple worlds started passing by as if it was searching for anyone best suited for the job the princess requested. Suddenly it stopped, as Celestia looked into the crystal, “What have we here?” she wondered, as she saw images flash by of four humans like Phoenix Wright using devices to subdue and catch ghosts, as a song played.

https://www.youtube.com/watch?v=2pDZpNNLXlM

She gasped, “These four seem to know how to handle the paranormal, but I must see more of these four if I am to truly call for their help."

Using the power of the crystal ball, she looked back on these four and saw three of them were actually professors at a university, until they were thrown out. They eventually went into business for themselves in paranormal investigation and elimination, with the help of a fourth member who took the job. She continued watching as the four took on various types of ghosts, including a giant creature made out of marshmallow, the boogeyman, and another evil entity known as Cthulhu. After seeing enough of the four, she smiled in confidence, “Yes. These four are exactly who we need to take on Sombra's ghost. I must also be sure to send a message to Twilight about this incident, but for now I think it's time to meet these... Ghostbusters.” she concentrated her magic as she was prepared to contact them.