u/Extra_Version8887

I'm on the second adventuring party about to watch the third episode so maybe this will be refuted, but my initial thought with the Underlord reveal was that his collection of dust includes the ashes from burned toys.
It was more common with scarlet fever (like Velveteen Rabbit) but it was still fairly standard practice to burn soft toys, bedding, etc when a child was extremely sick. It feels right to me with how much anger the Underlord holds. The like compressed and concentrated indignant rage of toys that were discarded and destroyed because they were loved.

It's not a bullet proof theory by any means, but I like it personally and would be interested to see what everyone else thinks.