Rewriting reality is being constantly done by Amberites - but rewriting all
of reality is supposed to require the Jewel.
But it doesn't.
Walk through Shadow, and you can change it. Those changes can spread to other Shadows - frex if you walk to a Ubiquitous item, you'll then be able to find that item in any other Shadow. That item can be anything
. Repeat this process multiple times, and you continue to change everywhere. Crunching the numbers, it would take you about 3 years to walk to items that can completely modify the Shadow they are in, and possess the Ubiquitous feature.
So you don't - you just walk the Pattern and teleport to the Shadow where they are!
Modifying the multiverse in this fashion will have been done multiple times in the past: the Borg from Star Trek and the replicators from Star Gate are the first two examples that came to my mind. Every time this happens, there will be a multiverse wide effort to repel the 'invaders' - and to locate and punish the perpetrator (assuming they aren't destroyed by their own creations of course - and remembering that they are years
This would actually be a reasonable way to start a campaign, as disparate PCs need to work together to save their Shadows; a good explanation for why
this collection of Chaos lords, Amberites and jumped up Shadowdwellers are even talking to one another, much less working as allies.
Continuing this thought: seeking to destroy
or depopulate the multiverse: imagine teleporting to a world which possessed an atmosphere which dealt Destructive Damage, and was (now) Ubiquitous throughout Shadow.
It's already happened!
The gas is called oxygen: the first mass extinction on Earth was the https://en.wikipedia.org/wiki/Great_Oxygenation_Event
. Assuming that Oberon's first family was anerobic, one of them could have decided to commit suicide, taking everybody with them, by this method - Oberon and Dworkin would survive, as they could shapeshift appropriately.
This would explain why there is always an oxygen atmosphere wherever you go: an unusual thing given you can't see
oxygen, so can't determine its level via shadowshifting. The multiverse is welcoming to oxygenbreathing life, due to an attempt to wipe out all life!