This was the first entry in the series where I finally understood what was going on with these portals. Maybe it was the repetition from the previous entries. After enough repetitions, my brain finally caught up to how your world-building was working.

I really enjoyed this one. It wasn't as simple as "portal opens, weird guy is there saying cryptic things, fin." Instead, there was interaction between the characters.

I will say that the dialogue seemed...off...in the early bits of your beginning bit. In particular, the way the dialogue was presented made it seem like the protagonist was unfamiliar with "The Those," even though he should be very familiar with them. He seems surprised by their existence, and this new portal guy seems to know all about them (which is fine, depending on how the universe works).

Also, and I cannot stress this enough: Dialogue attribution! The ping-ponging back and forth gets confusing. It works okay in a short segment of dialogue, and it works better when there are only two characters talking, particularly if each one has a very distinctive way of speaking. I bring this up now because you tend to shy away from dialogue attribution, and that's going to become too confusing as the cast of characters grows, if all these portals are connected to a single universe. Don't get ridiculous with it, though. It's not needed after EVERY bit of dialogue, and the good old-fashioned "said" is probably preferable to "pontificated, ejaculated, or interjected." Those ones are too distracting. "Said" strikes that middle ground where it keeps the reader oriented without drawing attention to itself. (I do think there's a place for other simple attribution words like "asked, shouted, screamed, yelled, etc.")

I'll get off my soapbox and see myself out now.

Oh, fun story, by the way!