The Karthakasel Divide, which includes the Osterbijan mountains and the Dacian hills, eventually pulls up into the huge mountain range to the East, that prevents all but the most determined traffic by foot.
There is no easy way over this mountain range, but there is a way through it. Or rather, under it.
Cross Way, as it is known in in the West, is a huge underground pathway. Part of the way must be undertaken by ship, through a large network of underground rivers. The rest is by foot, or mule. Obviously, the entire adventure is obscenely dangerous, and a host of terrifying legends of monsters and curses are associated with the place.
Cross Way is a subterranean route beneath the Karthakasel Divide, the huge mountain range that separates Karthack from Western Vosca. It is one of the few reliable trade routes to Karthack that doesn't pass through the lands of the Kedouins, or worse require going all the way around the Fang of Tujan to reach the Karthacki Sea.
The route was discovered during a Dacian mining expedition, and was opened while pursuing a formation of Orichalcum. The realization that the route led to Karthack was kept a secret until arrangements could be made with the Karthacks for trade. This is easily the shortest trade route to Karthack that exists (short of travelling through the incredibly treacherous steppe, of course) and it has proven a massive economic boon to struggling Dace, and is in many ways (supplemented by its output of ores and raw materials) the source of its rapid growth.
Cross Way itself is partially a Trackway system like those on the surface, and partially a rope-drawn waterway, with wagons transferring onto barges to be dragged across vast subterranean lakes, illuminated by the pale light of phosphorescent algae.
The dangers of Cross Way are numerous, and the sorts of merchants who travel the route are brave, brave men, willing to put their lives on the line for commerce and wealth.
So they're trade partners, but not terribly friendly ones. For one thing, there is a constant tension over who actually "owns" Cross Way. Dacian outposts in the underground have been creeping into Karthacki territory for years, and things have occasionally turned violent as the proud "Cross-Walkers" of Dace and the brave Kals of Kar-Ischil butt heads over patrol routes and territory.
There is also a degree of religious tension. The Dacians are primarily Genosian and Dessian converts in Pishka have stirred up trouble before. However, the simple logistics of Cross Way makes any sort of war close to unthinkable. Only a lunatic would try to transport an army through those deep halls.
Cross Way has become part of a trade route, jokingly called "The Black Path," that leads from Ruvia, through the black forests of the Kaselreich, through the rich lands of Kalmacia, into Dace, looping up briefly to the lip of Osterbija, and then plunging down to Pishka, and Cross Way. It emerges in Karthack, and continues East all the way to Kar-Karthack and beyond.
Rolling the Black Path is a euphemism for a journey that typically lasts over a year, a trade route from one end of the Path to the other, and back, trading as one goes. It is extremely dangerous, but incredibly lucrative as well, and many a brave band of young traders have made their fortune or died there.