The kings and merchants of Galar built a brutal, worldwide empire on the back of the tea trade. Farmers were forced off their land when it was converted into tea plantations, often to starve in the cities, for it was more profitable to grow tea than food. Slaves were worked to death processing sugarcane, soldiers and sailors lost to ensure the profit went into Galar's coffers. And the spirits of those so vanquished did not move onto the next world, but attached themselves to the beverage that meant their doom, creating the first Sinistea.
Sinistea seek revenge for their sad lives by fusing with teacups and, once used, transforming the tea into a sort of poison which allows them to steal souls. Today, this is an insignificant cause of death, almost treated as a freak accident, but in Sinistea's heyday it was common enough to give tea a dangerous reputation, akin to eating Qwilfish. Sadly, in an era where life was cheap and danger common, they could do little to blunt the drink's popularity. But the relatives of those who died of drinking Sinistea were not so easily mollified, and they led one of Galar's first great humanitarian movements, ultimately bringing about an age where tea drinking was no longer linked with human suffering.
It is no coincidence that true Sinistea are only found in antique teacups; none, thankfully, have been born in nearly two whole centuries. It is odd to speak of a species of ghost pokemon as "endangered", yet time leads all grudges to fade, even those exceeding a human lifespan. One hopes that future generations will know only of the false Sinistea, a harmless type of spirit interested only in pranks and petty mischief, and incapable of telling teacups made to look old apart from genuine antiques.