The Murdered World

The planet Gargannau was once a lush, green, highly advanced planet that was the home world of the saurian species, the Mogg'Theran. The Mogg'Theran gained almost all of their advanced tech from an ancient scoutship they uncovered on their world that belonged to an enigmatic race known as the Vorhasi, who some say are hundreds of thousands of years more advanced than any other known species.
The Mogg'Theran ruled over 200 star systems known as the Empire of Many Suns, with four races serving as vassal states for more than 300 years. They built the 10 Great JumpGates and connected their region of the galaxy. Then, about 200 years ago, the Vorhasi arrived at Gargannau and told the Mogg'Theran that they had five years to evacuate their planet before they returned to destroy it.
The Vorhasi had never been violent except to races that invaded their territory. Even then, ships found themselves disabled and wormholed dozens of light years back in their own territory. But everyone knew that the Vorhasi had the power to back up their threats.
The Mogg'Theran marshalled their fleet, a massive armada of thousands of ships, bankrupting their empire in the process in order to throw everything they had at the Vorhasi when they arrived. But just in case they failed, they evacuated everyone that was willing to leave.
Right on time, the Vorhasi arrived. One ship, barely large enough to be considered a cruiser, appeared at the edge of the star system. It sat there for about 30 minutes, then left. The Mogg'Theran thought it was a scout ship, and their massive fleet had scared it off.
An hour later, the molten iron core and all the geological and biological processes of Gargannau simply...stopped.
The core went cold. Life forms stopped processing energy. The temperature plummeted, and flora and fauna began to wither and die before the Mogg'Theran's eyes.
Panic ensued as the race feared that whatever was happening to their home world would spread. But it did not. Even the few Mogg'Theran on the surface who would not leave were apparently unharmed. But besides those few, nothing lived, not even bacteria.
The Mogg'Theran's empire, bankrupted and without their power base, fell as their vassal states revolted. The species, which was physiologically tied to their home world like Salmon are connected to their spawning ground, split up into massive nomadic fleets, some work for hire as mercenaries, some simply live on in roving fleets and a few others have departed for trips across the cosmos, fearing the Vorhasi may one day finish the job. The First Gate, once considered a wonder of the galaxy, floats dark, inactive and unmanned over a dead world.
Why did the Vorhasi destroy Gargannau? Nobody knows. And the Vorhasi do not explain themselves. They have not harmed a single being since then.
Some speculate that the Mogg'Theran were about to unlock some secret of Vorhasi technology that would have made them rivals, but the Vorhasi didn't destroy the scout ship. The Mogg'Theran took it with them, and all their research.
Some say that the Vorhasi were forcing the Mogg'Theran to evolve, by destroying the home world they were physiologically bound to by instinct. But why?
Others suggest that there may be another, more disturbing reason. There have been reports, scattered and unconfirmed, that Mogg'Theran psychics still on the planet heard something...alien...scream in their minds as the planet died. This has raised some questions that have no answers. Like what was a Vorhasi scout ship doing abandoned on Gargannau? What force could have disabled a ship by a species that can destroy worlds with no detectable weapon?
And why are there reports that occasionally Vorhasi ships return to Gargannau, scanning it...as though making sure the planet, or perhaps something that once lived on or under it, stays dead..........