Actually...to my knowledge, that's never happened.
Portal stones are used to close portals, not open them.
I'm pretty sure the idea that they could open portals started on other continents when they started showing up there, and eventually migrated back to BT. But there is nothing that any dev or GM has done (to my knowledge) to encourage this belief. It's purely a player invention.
Back when Vlaanderen was around and Arcane was the ruler, we opened a portal in Zisswii. Didn't really do much though. I guess it gave the daimons in the realm a few more soldiers, but that was about it. However, it still stands that we did open one, or at least gathered all eight stones to open one. Not sure how much game mechanics wise really happened, but Arcane at least roleplayed a portal being opened.
