A Question about Paracosms

From tech writer Gwern's list of open questions:

How common are, and what is going on psychologically, in the occasional eruption of large shared fantasy worlds (“paracosms”) among children and adolescents?

There are many cases of a (typically pubescent, typically female) child or adolescent building such an intense fantasy-world that they wind up sucking in and convincing friends/classmates. They typically go unreported except in extreme cases (such as the Parker–Hulme murder case, the Slender Man stabbing, the Manchester stabbing), often reported only in passing or via anecdotes—I have been told of 3 cases (2 from acquaintances, one indirectly), all of which follow the same pattern of a young female teenager building up a fantasy world (with heavy input from dreams) and engrossing friends/classmates.

But there doesn’t seem to be any recognized name for this pattern (“Tlön syndrome”? “Terabithia complex”? folie à plusieurs) or discussion of epidemiology. Is it an expansion of maladaptive daydreaming? Is prevalence underestimated due to childhood amnesia (similar to how imaginary friends are not anomalous but may be had by the majority of children, though they forget as adults)? Are the dynamics the same as proto-religions (the ways in which the paracosms are extended, particularly by dreaming, bear a great deal of resemblance to the origins of religions like Christianity)?

I am also fascinated by the question of belief and how it is shared, and in the relationship between beliefs that strike us as insane – e.g., if we stab this other girl Slender Man will appear and bless us – and those that seem more normal.