It is said that Serenians and Connlaothians share a common ancestor. Long ago, there was a great nation located beneath the Kilanthro mountains and far to the west. This nation thrived on magecraft and was a successful, prosperous place.
Cursed ruins to the north of Connlaoth, a scar on the land that birthed three nations...
That all fell apart when its mages got too curious and tampered with forces they shouldn't have, and in doing so unleashed a tainted, chaotic magic that became the nation's downfall. The surviving mages were able to contain that taint, but not before the damage was already done and the once great country was in ruins. The magic left a scar on the land and, to this day, the residual taint still clings to the ruins. Any mage who ventures there becomes infected with this chaos magic...and takes it with them when they leave.
While no one remembers the name of that nation, for this all happened so long ago, most know of its legacy and avoid the ruins at all costs. But there are those thrill-junkies, treasure-hunters, and knowledge-seekers that don't heed the warnings and instead choose to poke around the abandoned, crumbling buildings for artifacts and lost knowledge, and they suffer for it if they have even the smallest amount of magical talent.
The curse only infects mages, so those that lack magic cannot be infected. (For example, a shapeshifter cannot be infected or transmit the curse unless their shifting is a magical ability; a magically created creature cannot be infected or transmit the curse unless they possess magic of their own.) But for those who possess even the tiniest amounts, the effects are disastrous and affect not only the mage, but the world around them.
The effect on the infected mages is a loss of control of their magic. Spells go wrong, backfire, and cast without them meaning to. But worse still is the effect on the world around them. Wherever they go, they spread entropy and chaos. People go mad around them; animals go rabid; nature twists, rebels, or dies; magical enchantments and spells warp and go wildly wrong...and the effect gets worse the longer they stay in one place so they have to keep on the move constantly.
To make matters even more complicated? Any mage the infected mage comes into physical contact with also contracts the curse.
It acts as a magical plague, spreading from mage to mage like wildfire. While one infected mage may be but a nuisance, a group of infected mages could crumble an entire nation the same way it destroyed that long-dead civilization.
While there is a cure, it is difficult to obtain and the only quick and surefire way to stop the spread is to kill the infected mage(s). In Adela, infected mages are even expected to commit ritual suicide for the good of their community. In cases where outbreaks have gotten especially bad, some nations have been known to request aid from Connlaoth, contracting the Mordecai to help stamp out outbreaks.
Magical items are immune to infection; while there may be cursed objects inside the ruins themselves, the curse is only transmitted between living beings. However, magic items are still subject to the affects of the curse if an infected mage is in range. That means the item may go haywire, backfire, have unexpected effects, cast without meaning to, and other such effects. These effects will wear off once the infected mage is out of range.
The Order of the Moon is an ancient, demon-hunting unit based in Moonspear that started back when Serendipity was first formed. They helped fight and purge the plague when it first swept through the land. Now they guard a fragment of the curse, keeping it locked deep in the center of Serendipity. They keep this part of their order's duty a secret to outsiders. Their years of intense training and prayer have given them an immunity to the curse and a few spells that can help contain the curse.
However, the immunity is difficult to earn and there are too few of them to do much good if the curse ever spreads to a large number of people. Members of the order consider it their duty to purge or destroy anyone carrying the curse.
Priestesses at the temple in Moonspear discovered that, with the heart of a powerful demon, they could transfer the curse from the infected person to the demonic heart and use the water from Moonspear's sacred spring to purify the person. The curse itself could then be contained by placing the heart in a lead box filled with water from the sacred spring box and sealed with wax and runes, and it could be destroyed by throwing it into the Thunderblacks' active volcano.