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The question is a little ambiguous. Some thoughts:
It's possible they visited an actually-real Dream Land or Magical Land via their dream. A particularly widespread Shared Dream could also be pretty real (i.e. all the participants are real people, so it's at least as "real" as playing an MMORPG together).
If they're just confused and thought they were dreaming when really they were awake, then That Was Not a Dream. Also Mind Screw may apply if it's not even clear to the audience whether the writer is dreaming, awake, or seeing real or unreal things.
It's probably otherwise going to be hard to find a single trope that is this specific.
A trope in which a writer creates a content based on their dream, unbeknownst to them that it's real.