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Prompt: Write a story about the dust in Uncle Andrew's box or the people who had it, at some point in its history.
In the dawning of each world, there were many chinks and chasms that permitted passage between it and others. They grew rarer as the world went on and established itself. Omravi was old, lit by a perpetual moonlight, and few remained.
The Omravians by this time were a skilled and magical people. They had studied the chinks carefully, and from them had learned many things, such as the art of ripping their world's space to move great distances quickly. But they could not create new chasms, could not rip their way into other worlds, and so over time they taught themselves how to stabilize those passages that were left.
Each passage was considered worthy of this preservation by its very existence, but as may be expected, some were more attractive to the Omravians and some less. One in particular they thought quite useless and maintained solely because the law required, for it led only into a endless forest filled with pools of water. There was a peculiar property in the air of that world that induced lethargy and forgetfulness, and after earlier generations had nearly lost three excursions to this malady, they were content to simply let it lie unexplored.
Of course, this danger had only made it tantalizing to younger Omravians, and "bring me something from the Dead Forest" was a common dare among the more reckless sort—at least until the day that an entire class of youths went to the Dead Forest together as a lark, and not a single one ever returned home. The passage was maintained, but after that day few even of the young considered visiting it at all.
But the party that had been lost to the Forest had not been lost entirely. In truth, they had not even been lost to the Forest. They had gone in and come out safe with their souvenirs, but then by some quirk of fate (or perhaps by the hand of the Three, depending who you asked) had stumbled through a heretofore-unknown chasm, and stepped out from the cool moonlit glimmer of Omravi into a hot Mediterranean sun.
It was hard when they realized the chasm had closed behind them, but as youths raised in a world well-accustomed to such chasms, they dealt with this rather better than some worlds' children would have. They made their home on that large island, acclimated to the surroundings, and found they were like enough to the people of that world that they could intermarry. They built a society informed by their Omravian origins, but with a language that melded the two most in use by their neighbors. In that tongue they called their land Άφαλονα, Hafalona, "beneath the moon", in memory of their lost homeland.
Their land was strong and wise, and their people remained magically gifted, even in a world where magic was much lessened. As they studied, they found that the things they had brought from the Forest allowed them to build far beyond the abilities of the surrounding countries. These, therefore, they used as sparingly as they could.
When there remained but one small box of dirt, they deemed it beneficial to dissolve the island entirely and disperse into the rest of the world. In later years they would be immortalized in song and story both as Avalon and Atlantis, though neither bard nor historian would correctly record the location of their vanished island, and no archaeological expedition would ever unearth its remnants. They knew their craft too well for that.
The final box itself was passed down as an heirloom. Father to daughter and mother to son in the early days, aunt to niece and uncle to great-nephew as the Omravian lineage slowly died away in the latter ones, when all trace of its true origin had been forgotten. Down through the centuries and millennia it passed from descendant to descendant, until it found its way into the hands of one of the last, an English woman with no children who had married a man named Lefay. She did not make use of it—she was still wise enough for that, despite other follies—but neither could she bring herself to destroy it, and so entrusted that task to her godson upon her death.
But what came of it when he did not obey, and the use to which he put the dirt...well, that's another story that's been told in other places.