Wardstone Covenant

When the people of the Midnight Expanse discovered the wardstones’ ability to keep the demonic hordes at bay, they began settling down in more permanent homes around them. The three that initially found these artifacts became three kings of their respective peoples, believed to be blessed with guidance from the gods to the instrument of their salvation.
  When these settlements grew into proper cities, and the rarity of wardstones became more apparent, some began to covet the wardstones of the other cities, to expand their territories and bring all the warded zones under one unified banner.
  Conflict arose and burned hot, but this divided the people’s energy to fight off the demon hordes. It cost many lives for no gain, as the wardstones were too far apart to travel between in the span of daylight hours, leaving any invading force at the mercy of demons with only feeble torches to protect themselves.
  When pride and greed fell through to this needless loss of lives, the founders came to an agreement to aid each other rather than destroy themselves.

The covenant is fairly loose with its terms, as the initial kings were unwilling to give up their personal freedoms over their cities. The initial agreements were merely a ceasefire, and trade agreement to foster friendly terms between the three cities.
  It was considered to make the city of Luxinae a city under all their collective control, making it perhaps a trade hub between their cities as it naturally sat in the center of the savanna rifts. However, ultimately it was decided that the city was an ill-omen, haunted place that should not be resettled, lest its curse spread farther than its own walls.
  Every year, representatives and traders from all the allied cities will gather outside Luxinae to discuss diplomatic matters, kindle bonds between the peoples of their cities, and discuss any changes that need to be made.
  It is against the laws to deny a citizen the shelter of a city’s walls and wardstone as night approaches, as all of the people should be entitled to the protection from the demon hordes. Unfortunately, this protection does not extend to exiles or the desert tribes that share the Midnight Expanse.
  When the initial kings died, it became needed to choose new leadership and for them to sign the covenant. It was decided that, instead of multiple kings, that the cities will be led by a Royal Advisor, and the eldest leader will be the new King. The advisor is selected by vote of the civilians, but the king has the final pick of the candidates they offer.

Every three years, the covenant is reforged in metal, with any changes that were required. To limit single party corruption as much as possible, changes are subject to the vote of all those who attend the gathering, not just the king and advisors.