Portals to the Mists
I think there is some confusion here about the kind of ways people contact and travel through the Mists. One of the first instances of humans traveling through the Mists is I think Lord Odran, and there are far more elaborate lore works on him than I can recite from the top of my head.
But what we see in Temple of the Ages in GW1 is entirely different. Here we gain the favor of the Gods, and they allow us entry to the Mists.
The Asura have expanded on much older portal technology (of unknown origin I believe) and made it their specialty. As far as I know, only Asura are able to construct these typical gateways that use the Mists as a conduit to go from one place to the next. Fractals also shows us that Asura can manipulate the Mists much further, but Dessa seems unaware, or unwilling to see that there is a portal from LA straight into her lab. This is something the Consortium is responsible for, but it’s clearly all Asura technology. The Fractals lab itself is something entirely new though, and not something we’ve seen before. Lets also not forget the Thaumanova reactor, which may yield even more surprises in terms of Asura manipulations of the Mists. But that depends on the outcome of the election.
We have seen Norn Shamans summon creatures forth from the Mists, but this does not mean the gateways go both ways. In fact, if players try to enter them, this is impossible. I’m not sure if we can consider this then as a lore fact, since it may simply be a game limitation. I would assume since it is a clear summoning, that this is different from actually creating a stable two-way gateway into the Mists.
When human priests summon the dead or talk to them, again they contact a soul from the Mists, but they do not open a portal. I would assume Sylvari and Charr can also perform such rituals, but it is probably uncommon.
To me this still indicates that only Asura know how to create portal travel, and only the Gods can allow us entry otherwise.
Well… unless there is of course a rift, like at the Tomb of the Primeval Kings.
(https://www.youtube.com/watch?v=D-On3Ya0_4Y)
Minor correction: when human priests summon forth souls, there is indeed a portal. Still, that is likely linked to Grenth’s power, and not the priest’s.
Also, norn havrouns are indeed able to take other people with them through a portal into the Mists. Although, again, this would likely be power from a Spirit of the Wild and not the havroun’s own.