The Living Concoction
Everything goes in the pot, a non-stop bubbling brew. Everything vile, nasty, evil, disgusting, toxic, acidic or deadly. Everything that crawls, flies, skitters, hops, claws, screeches and roars. The pot cares not, it accepts one and all.
Sorcerers, alchemist, druids, necromancer, witches and wizards will add into a pot. Constantly refining the ingredients, adding a pinch of this or a drop of that. Always with a goal in mind and a limit set.
What then happens to the foolish, the young, the experimenter or simply the mad. What happens on the rare occasion when a pot is pushed beyond its limits. A cauldron collapses in calamity. An alchemical stew surges upwards. It consumes, and it consumes everything in the vicinity with an titanic explosive force like a magical Hiroshima.
Once within a millennium something rare will happen. The consumed humanoid will be absorbed by the explosion but will also bond with the concoction. Becoming a living, sentient goo. An alchemical abortion that takes small flecks of everything that was placed within the cauldron. A living breathing cancer upon the land.