Cave Portals

Fantastic pheonomon
If you pass through them, they transport you to the south, inflicting 5 Stress. This is one-way.
The Xanthic Hold was originally built as an outpost to guard the Cave Portals from beastfolk attacks in the early days of the settlement of the area.
The Sprongindilfin don't know about the cave portals.
The monks of the Firdt Lake Shrine used to utilize the portals when they headed south at the end of their shift at the shrine. They remember there was an important detail about the portals, but only searching through the archives at Far Abbey will reveal that they knew about the one-way condition (not how to get around it).
If you're heading to The Braystones, you can use the south portals to go north.
On a wall in the The Nekromant's Tomb is carved the plans to use the portals to move the army quickly, launching a surprise attack on Prensuchdt