Arrival in Sigil

To arrive in the City of Doors, one must only need to find a “door” to step through. Sounds easy, right? It certainly can be! For some, it's accidental. For others, it's a planned journey. Sigil attracts all kinds of people from all kinds of planes, being the center of the Multiverse and a hub of interplanar activity. While the city's stable population averages something around a half-million souls, it's not unusual to have over a million people within the city at any given point in time.

Journey from Alderheim

An excerpt from Ohza's Abridged and Annotated History of the Material Planes, Volume Seven:

The land of Alderheim had seen too much in too little time. Those who sought power, sought change, sought transcendence wreaked havoc by tapping into magic too far above their own. In their short-sighted folly, they shook the world and rained destruction down from their lofty seats in their ornate capitals. Thousands upon thousands of voices cried out as the cataclysm took their homes, their livelihoods, and their lives, leaving the remainder to mourn and try to piece together a future with the ashes of the past.

The next few hundred years were no better. There were some that fancied themselves the true Power of this plane, seeking to finish what their ancestors had started, reaching to tap that potential that lingered just out of reach. Others organized to oppose them, thwarting many attempts to recreate what should have gone right. Still more fought for just one more day on this land, creating their own path forward and hammering out every stone on their journey. The latter were the most taken by surprise when the ground began to tremble again.

Great cracks ripped their way across the surface of the planet, spewing noxious fumes and a corrosive ichor that corrupted anything it touched. Sadly, journeying into a realm yet undiscovered and putting down the tyrannical beast with its machinations was only enough to buy a few weeks of silence. Once more, the ground began to shudder, churning so violently that one might think the planet was rejecting life, itself.

With no focal point for this mess, nor any plan to fix it, the denizens of Alderheim clutched each other and braced for the worst. When all seemed lost, one last rip in the Veil tore open and a single light of hope appeared in the form of a massive, glowing portal. A solitary figure stepped through wearing tattered clothes and sporting an expression of both awe and bemusement as he took in his surroundings. The man's voice rang out over the crowd of those that had gathered.

“Oi, berks! Git yer jink and blitz, lest ye want to start countin' worms!”

Admin notes:

Thanks to Sigil being a setting that is accessible by any race from any plane, there is no reason to change or omit parts of a character's backstory if a player is porting them from another sim. In fact, it's perfectly reasonable to assume they can (if they have a key to that portal) freely hop to and from that sim if they so choose - excluding Alderheim, of course.