The Tide Cults are a whispered phenomenon in the Isles, not one exclusive to Xian. Yet the sheer mass of people within the city walls is an inviting target to the sinister forces of the Tide, and with so many ears to hear, a few are certain to answer it.
The cults invariably begin with dreams. A receptive mind is found and regular visions of power and glory are impressed into the sleeper's thoughts. Most throw off the influence, dismissing it and remaining on the virtuous path out of either fear or honest integrity. Others, however, are more intrigued- and to these a few small rituals are taught by smiling dream-companions.
These rituals are petty at first, conveying only shreds of good luck, health, foresight, or prosperity. Soon it becomes clear that more people are needed for larger rituals, and the cultist entices others to join him in these 'foreign rites'. Others may or may not realize their situation, but by the time the rituals graduate into blood-drenched orgies or sadistic human sacrifice, there is no doubt that the acts are those of unspeakable evil.
Even those few with a shred of conscience left do not dare expose the cult for fear of their own guilt. Eventually, if the cult is not stopped, the rituals will tear open a path for the Red Tide. Water turns to blood around the ritual site and a red mist brings demons from the forbidden fane. Sometimes the cultists are spared- and sometimes they reap a full measure of what they have sowed.
The Red Tide cannot linger unwelcome, however, and if the cultists can be slain the mist must evaporate and the crimson pools return to plain water. On at least a dozen occasions since the founding of the city, Xian has tasted the attentions of the Tide. The city watch and brave adventurers managed to quell it before it became more than a horrible rumor and an unexplained mass murder or three, but there is always the danger that one day a cult will form that is large enough to hold the door open for a true army of demons. And who is to say what is happening out in the countryside, in the quiet villages where no magistrate goes?