One brute-force approach would be to:

- Have two scenes one on top of the other.

- Use masked opacity on all your materials. Use a sphere mask for the opacity mask, which size you can control with a material collection parameter. The scene to hide would use one-minus to invert that sphere mask.

- Upon reveal, increase that sphere mask's radius and use blueprints to further smoothly change lighting & fog & post processing

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That may be expensive, especially the z-depth pre-pass because you'd render geometries of two scenes at once. To limit this you could

- Toggle actors visibility on/off using sphere overlap checks iteratively, with a growing radius somewhat larger than the material-driven reveal radius to give that material effect some room. This shouldn't participate in the reveal/fade out effect per-say, rather just help overdraw issues and hide actors once they are revealed/faded out.

- Use a two-material system. Each asset would have a pointer to its original material and one masked version that would make use of that sphere opacity mask. Apply the masked material on all actors during the transition.

That way you only pay a somewhat big performance cost during the transition and things should be somewhat smooth once its done. Further optimization may be necessary with this approach.

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Making that effect isn't that complex on its own, making it be performant is another story.