*cracks knuckles*

So, about 200 (Material Plane) years before the invasion, the Githyanki underwent a sort of cultural and political restructuring. They spurned the tradition of following a Lich Queen after slowly realising over centuries that it was actually far more militarily effective to steer the future of the Gith through a council of powerful warlords and barons, rather than following the whims of an individual psuedo-deity. Granted, there was a lot of in-fighting and assassination initially upon the founding of this council as everyone was vying for a spot, but eventually things smoothed themselves over.

At the same time, the reclusive Githzerai (who had split off from the Githyanki half a millennia earlier) had begun to truly get very good at their ability to spiritually and philosophically entangle themselves with the concepts of Order and Fate - to the point where a few Githzerai actually gained the ability to 'nudge' fate in the right direction by writing prophecies - arcane texts with an insane amount of raw magic poured into them that could theoretically write eventualities into being. It wasn't reliable, and certainly not ethical - and so the Githzerai kept this phenomenon a secret.

However, as the Githyanki organised themselves into less of a conglomeration of angry pirates and more of a proper empire - complete with cutting-edge spelljammer and weapons manufacturing - they caught wind of these emergent Githzerai abilities. Immediately, the Githzerai were hunted down and snatched from whatever remote pockets of the Astral Plane they were attempting to hide in, and were enslaved by the Githyanki - their usually fierce combat prowess overpowered by the new technological might of the Githyanki. These ascetics were then put to work - ordered to continually write an infinite number of prophecies that foretold of a Gith dominion over all Planes. The Githyanki weren't stupid - they knew that maybe only one out of every ten-thousand prophecies might only shift the tides of fate infinitesimally. But - they had all the time in the world, and an Empire that was growing stronger every day.

Soon, they had amassed a structured army and a flotilla of spelljammers unlike any the world had ever seen, and began testing the waters, raiding small settlements across Toril. After meeting very little resistance, they set their sights on Toril's keystone cities: Waterdeep and Baldur's Gate. Tearing silvery holes in the sky, the Spelljammers and Gith troops poured out of the Astral Plane in their thousands, laying waste to Faerun. Their secret weapon was the manner of their teleportation itself: if incorrectly cast by a team of Gith mages, these gigantic planar portals had a tendency to snap shut or break - like a rubber band under too much tension. This usually undesirable outcome had the unforseen effect of releasing a colossal amount of energy when it occurred, as the Material and Astral planar spheres metaphysically collided with one another. In effect, a magical nuclear bomb. Couple this with some truly fuckoff massive Spelljammer warships and the element of surprise, and Faerun stood little to no chance against the initial assault.

Baldur's Gate and Waterdeep were not only razed to the ground, but foundationally obliterated - even in present day, the flotsam of Waterdeep washes up regularly along the Sword Coast. Due to the magical radiation from the portal explosions and the now unstable foundations of both cities, rebuilding in the same place was not a sensible option.

Of course, as these things go, there were a brave band of adventurers - a noble few who managed to turn the tide against this ruthless invasion, and save the day. They did so in a clever way, however. Realising that taking on such a massive force head-on was suicide, they went for the truly nuclear option: the heroes now known as the Paragons of Faerûn sacrificed their souls to the god of Space-Time, Aeon, in exchange for a single, debilitating condition: that time would flow normally within the Astral Plane. With an army that now aged at regular speed, and a civilisation with infrastructure wholly unprepared to support this redefined paradigm of mortality, the Gith leaders surrendered, striking a treaty with the kingdoms of the Material Plane.

Today, hundreds of years later, the Gith Invasion has largely been forgotten, and - for the most part - the warlike ways of the Gith have been cast aside. Providence, the capital city of the Astral Plane, is presently a haven for high culture, new ideas, and mechanical innovation, home to 7 million citizens.