Five Shires

The Fives Shires were a lush and tranquil halfling nation on the world of Mystara, that ran northeast along the base of a mountain range called the Black Spires. It was also known as Hinhome, for the halflings of Mystara referred to themselves as hin, much like the halflings on Toril. Beyond the name similarity, the hin of these two worlds were said to have much in common.

Cosmography
Hidden deep beneath the Black Spires was a circle of Blackflame, a magical fire-like substance, and it acted as a portal to the halfling nations of Luiren on Toril and Green Fields on Oerth. More specifically, this portal would dump one out somewhere in the northeastern reaches of Luiren. In the Present Age, it was unknown if the portal still existed as of the late 1400s DR.

History
During a time of pestilence and savage warfare against orcs, the hin were forced to go into hiding underground for protection and in the process broke into tunnels that predated their own, wherein they discovered the Blackflame. It was found by a maiden beneath the Black Spires, burning upon amethyst veins along the shores of an underground lake, who touched it and found it did not harm her. The Blackflame served as a rallying point for the hin, igniting newfound hope for the future.

In later centuries, the Blackflame would become a cornerstone of the Shires' clans and their sacred mysteries faith, with each clan stronghold keeping a crucible of Blackflame within a sacred place and two to three "back-ups" hidden underground. The hin were very secretive regarding the substance and its many powers, such as a way known only by a select few clan leaders for how one could create their own rings of blackflame to other worlds.

Thus, most travel between Toril and Mystara was by halfling merchants traversing between Luiren and the Five Shires. The Minstrelwishes, a mercantile family of Torilian halflings that collected lore on portals and were interested in planewalking, were also found in Mystara and the Five Shires.

Background
The Five Shires (1988), a dedicated gazetteer for the eponymous nation, was notably written by Forgotten Realms creator Ed Greenwood. It was while writing this book that Ed coined the term "hin" for halflings, which he would later re-use for the halflings of Toril. In addition, over the years Ed has implied that the Five Shires may have served as inspiration for Toril's own halfling homeland of Luiren, or vice versa. Ed has also mentioned that while developing the Five Shires he tested out the magical substance Blackflame in his Home Realms campaign, but has not given any indication that it can be found in the Realms proper.