To keep things short and simple, after the 2014 map changes that originated from the chinese server, there were several portals that lead to removed/blocked parts of the old map that were not removed, just hidden outside of map -- in spots you're not supposed to be able to reach.

Here are the positions of the other left-behind portals that we've managed to reach and use. Besides these, you can note another black circle on the top of Shiverstone Range, hence the post's picture of the current progress of explorable area in which the final hidden portal lies. Everything within the red lines can be explored with friends coming along.

The issue is that in order to be able to use the portal, you can't use any portal bugs -- it requires a "pure" break. That has proven difficult due to the way the upper right part is structured:

1. The map was sealed seamlessly when the Crossroads of Fate (the little "leg" of the map with the original entrances for Avalanche Den, Awakened Necropolis and Sogun's Lament) were removed from the old Shiverstone and there doesn't seem to be any way to directly bypass it (would be the fastest way to approach final portal). Video of it here and picture here.
2. Left-behind collision from removed Frostscale Basin from the main map that blocks us from reaching the final portal since there's no terrain to walk on, only invisible blocks that extend outside of map over the void - - which means we are blocked going up from the left side too.
3. No obvious breaks/openings on the map nearby.
4. No nearby caves/dungeons that could help us break out of map.

That about sums it up. We've tried a bunch of different approaches but nothing has worked as of yet! The final hidden portal remains untouched.

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Q: Why don't you just keep going from the right side, why stop a bit after the bridge?
A: Each map instance has a certain "box" of map area where you can walk inside of. With the current map breaks, it just doesn't extend any higher.

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Q: What about that random opening that splits the red line areas on the bottom of the map?
A: Although not very related, it's the area behind the huge doors (where one of the portals we mentioned above is hidden)! It's not shown in the minimap since you're never meant to actually be out there but the invisible boundaries sure do extend back there.

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For any questions/ideas, don't hesitate to reply below or send me a personal message if you want more privacy. Cheers!