From Introduction:
Interplanar highways stretch between the worlds of the multiverse, built by an ancient empire hundreds or thousands of years ago. That civilization is gone now, but the roads they left behind still remain. Not only that, they are still used, though far less frequently than in the past. The occasional party of wizards, faeries, or planeswalkers have found them to be a useful shortcut, and they have even been discovered by a few arcane mercantile operations. But no one has yet exploited them to the same extent as those who made them. Perhaps some intrepid adventurers could sell their knowledge to the highest bidder, so that newer empires might expand their reach across the cosmos.
Of course, the threat of banditry is ever-present along any road, even ones that pass between the stars. The builders of these highways stationed numerous guardians and protectors along them, but only one of them remains: the star dogs. These immense and oddly-featured canines patrol these highways incessantly, ready to pounce upon anyone they encounter engaged in crimes. It is the oddest feature of these ancient trade routes: though they pass through realms full of danger, they are in fact much safer than many of the roads you take back home, where only the nobility or the soldiers of the king will save you from the edge of the highwayman’s blade.