Well, using reverse-image-search on your reference, here's a video of the effect (at 8:50) https://www.youtube.com/watch?v=jdD8zDbp2Qo&t=530s This uses a mask inside the "portal" to blend with a cubemap, or if you want to get more advanced and have a "live" view of the next level then you can render the next level to an…
Small update / extra question: Thanks again for the suggestions. I looked at a few portal VFX tutorials, but most of them are more focused on Niagara particles or gameplay portals. What I’m still struggling with is mainly the material/shader structure itself. Right now, my biggest issue is that the portal still feels too…