To my shame I do not own Harry Potter nor Stargate I am merely playing in their sandbox.
Ten thousand years ago at the height of a great war that had lasted for over a century the ancient and enlightened race called the Lanteans were preparing what they hoped would be the foundation for the salvation of their race. During the war with the Wraith an insecto-humanoid species that fed upon the life force of humans and Lanteans alike, it had become apparent to the Lanteans that they could win almost every battle and engagement but they could not see a way to truly win the war.
In a last ditch effort to save their race and the people of what would one day come to be known as the Pegasus galaxy the Lanteans decided to relocate their largest city ship Avalon named for their ancient home the Avalon galaxy back to their former capital world Terra which had be long abandoned due to the ravages of a deadly plague many millions of years before. The city ships the Lanteans created where a wonder of sophisticated architecture and technology as they truly were the most advanced starships ever created.
While Atlantis was the crowning glory of the Lantean Empire being the first city ship built it was Avalon that was the jewel in that crown. Being three times the size of Atlantis capable of housing millions of people it was the height of technology. Completely fitted out will all of the Lanteans latest technology and a knowledge database spanning tens of millions of years. It was the plan of the Lanteans that Avalon would return to Terra and in doing so it would create for them a location outside of the theatre of war with the wraith that would allow them to build up forces and effect repairs to damaged ships with fear of attack but most importantly it would provide them with a safe staging area that the wraith could not reach thus allowing them to prosecute the war more effectively.
As preparations where made for the departure of Avalon back to Terra the wraith discovered what the Lanteans were planning and assembled a massive fleet to try and prevent Avalon from leaving. What the wraith did not know was that the planet Avalon was leaving from also housed an experimental weapon called Arcturus. As the enemy fleet encroached Arcturus was activated and began to decimate the wraith fleet. With barely more than a skeleton crew on board and refugees from the Furling supernova disaster Avalon lifted effortlessly from the planet's surface. Unknown to those on board Arcturus had spun out of control and was beginning to flood the planet with highly dangerous exotic radiation.
As Avalon moved into high orbit Arcturus began to target and fire at the great city-ship. With its immense shields the city was safe from damage however the exotic radiation saturated those very same protective shields and as the ship entered hyperspace the interaction between the radiation and the hyperspace window caused the ship to be thrown off course and into another reality. As Avalon left hyperspace at its supposed destination the crew realised something was wrong when they could not detect the outpost they left behind when they abandoned the planet nor the astria porta that should have been present. It was Terra but not the one they knew.
Alarms sounded throughout the city as radiation started to penetrate the shields. In order to save everyone the commander of the city ordered emergency beam out of all personnel to the planet's surface. To protect the life on the planet the city was ordered to using its phasing cloak to move into a pocket dimension and await further orders.
As time went on the evacuees settled into their new life and their descendents grew and prospered in numbers gradually over the millennia they laid the foundations of what would become the wizarding world. Over time the descendants forgot their own history and Avalon become a mythical island where magic came from.
Of the original crew and refugees from Avalon they lived out their lives and choose to either ascend or pass into death depending on the situation. There are a few who ascended and took up the mantel of watching over their children hidden, watching and waiting….
In the city of Avalon a council of six ascended beings were having a discussion about events in the wizarding world when an alarm sounded. Something that had never happened before in the ten thousand years that Avalon has been waiting took place….The gateway activated.
A man who looked to be in his mid forties with piercing blue eyes and pepper gray hair eyes widened and his head snapped around towards the location of the gateway. "Quickly my friends this may be what we have been waiting for" he said.
With that the six beings disappeared in a flash of white light only to reappear at the location of the gateway. When they reappeared they were surprised a man lying unconscious on the floor. One of the women knelt beside the body of the man and reaching out with her senses assessed the health of the as yet unidentified man. Gasping in recognition she turned to face her companions "Well isn't this interesting….Sirius Black".